Analog circuit and semiconductor device

ABSTRACT

An object is to obtain a semiconductor device having a high sensitivity in detecting signals and a wide dynamic range, using a thin film transistor in which an oxide semiconductor layer is used. An analog circuit is formed with the use of a thin film transistor including an oxide semiconductor which has a function as a channel formation layer, has a hydrogen concentration of 5×10 19  atoms/cm 3  or lower, and substantially functions as an insulator in the state where no electric field is generated. Thus, a semiconductor device having a high sensitivity in detecting signals and a wide dynamic range can be obtained.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.16/133,823, filed Sep. 18, 2018, now allowed, which is a continuation ofU.S. application Ser. No. 15/652,312, filed Jul. 18, 2017, now U.S. Pat.No. 10,115,743, which is a divisional of U.S. application Ser. No.15/063,706, filed Mar. 8, 2016, now U.S. Pat. No. 9,716,109, which is acontinuation of U.S. application Ser. No. 14/339,540, filed Jul. 24,2014, now U.S. Pat. No. 9,419,020, which is a continuation of U.S.application Ser. No. 13/733,291, filed Jan. 3, 2013, now U.S. Pat. No.8,803,589, which is a continuation of U.S. application Ser. No.13/568,186, filed Aug. 7, 2012, now U.S. Pat. No. 8,350,621, which is acontinuation of U.S. application Ser. No. 12/907,559, filed Oct. 19,2010, now U.S. Pat. No. 8,242,837, which claims the benefit of a foreignpriority application filed in Japan as Serial No. 2009-242853 on Oct.21, 2009, all of which are incorporated by reference.

TECHNICAL FIELD

An embodiment of the present invention relates to an analog circuitincluding a field effect transistor in which an oxide semiconductor isused. Further, an embodiment of the present invention relates to asemiconductor device including the analog circuit.

In this specification, a semiconductor device means all types of deviceswhich can function by utilizing semiconductor characteristics, and anelectrooptic device, a semiconductor circuit, and electric equipment areall semiconductor devices.

BACKGROUND ART

A technique for forming a thin film transistor (TFT) by using asemiconductor thin film formed over a substrate having an insulatingsurface has attracted attention. Thin film transistors are used fordisplay devices typified by a liquid crystal TV set. As a semiconductorthin film that can be applied to the thin film transistors, asilicon-based semiconductor material is known, and as another material,an oxide semiconductor attracts attention.

As a material of oxide semiconductor, zinc oxide or a materialcontaining zinc oxide as a component are known. Further, a thin filmtransistor formed using an amorphous metal oxide (an oxidesemiconductor) having an electron carrier concentration less than10′/cm³ is disclosed (Patent Documents 1 to 3).

REFERENCE Patent Document [Patent Document 1] Japanese Published PatentApplication No. 2006-165527 [Patent Document 2] Japanese PublishedPatent Application No. 2006-165528 [Patent Document 3] JapanesePublished Patent Application No. 2006-165529 DISCLOSURE OF INVENTION

However, a difference from the stoichiometric composition in an oxidesemiconductor arises in a thin film formation process. For example,electric conductivity of the oxide semiconductor changes due to theexcess or deficiency of oxygen. Further, hydrogen that enters the oxidesemiconductor thin film during the formation of the thin film forms anoxygen (O)-hydrogen (H) bond and serves as an electron donor, which is afactor of changing electric conductivity. Furthermore, since the O—Hbond is a polar molecule, it serves as a factor of varyingcharacteristics of an active device such as a thin film transistormanufactured using an oxide semiconductor.

Even when the electron carrier concentration is less than 10¹⁸/cm³, theoxide semiconductor is substantially n-type and the on/off ratio of thethin film transistors disclosed in Patent Documents 1 to 3 is only 10³.A reason of such low on/off ratio of the thin film transistors is highoff-state current.

A circuit formed including a thin film transistor with low on/off ratiotends to operate unstably. In addition, in the case where such a thinfilm transistor is used in an analog circuit, an enough dynamic rangecannot be obtained. Further, high off-state current causes a problem inthat the detection sensitivity to weak signals cannot be increased.Furthermore, high off-state current also causes a problem of unnecessarycurrent flow, which increases power consumption.

In view of the above-described problems, it is an object of anembodiment of the present invention to reduce malfunction of a circuitincluding a thin film transistor which is formed using an oxidesemiconductor.

Another object of an embodiment of the present invention is to increasethe dynamic range of a circuit including a thin film transistor which isformed using an oxide semiconductor.

Another object of an embodiment of the present invention is to increasesensitivity in detecting signals of a circuit including a thin filmtransistor which is formed using an oxide semiconductor.

Another object of an embodiment of the present invention is to reducepower consumption of a circuit including a thin film transistor which isformed using an oxide semiconductor.

In an embodiment of the present invention, an analog circuit is formedwith the use of a thin film transistor in which a channel region isformed using an oxide semiconductor which is an intrinsic orsubstantially intrinsic semiconductor and is an oxide semiconductorhaving a larger energy gap than a silicon semiconductor, which isobtained by removing impurities (hydrogen, moisture, hydride, hydroxide,or the like) serving as an electron donor in an oxide semiconductor.

Specifically, an analog circuit is formed with the use of a thin filmtransistor in which a channel region is formed using an oxidesemiconductor in which hydrogen or OH group is removed so that hydrogenbe included therein at lower than or equal to 5×10¹⁹/cm³, preferablylower than or equal to 5×10¹⁸/cm³, and further preferably lower than orequal to 5×10¹⁷/cm³, whereby the carrier concentration becomes5×10¹⁴/cm³ or lower, preferably 5×10¹²/cm³ or lower.

Impurities such as hydrogen which forms a donor is reduced as much aspossible so that the energy gap of the oxide semiconductor is 2 eV orlarger, preferably 2.5 eV or larger, and further preferably 3 eV orlarger, whereby the carrier concentration is set to be 1×10¹⁴/cm³ orlower, preferably 1×10¹²/cm³ or lower.

By using the purified oxide semiconductor for a channel region of a thinfilm transistor, even when the channel width is 10 mm, a drain currentof 1×10⁻¹³ A or lower within a gate voltage range of −5 V to −20 V canbe realized in the case where the drain voltage is 1 V to 10 V.

An embodiment of the present invention is an analog circuit whichincludes: a reference transistor; a mirror transistor; and a detector.The reference transistor is electrically connected to the detector. Adrain and a gate of the reference transistor are electrically connectedto each other. The gate of the reference transistor is electricallyconnected to a gate of the mirror transistor. The reference transistorand the mirror transistor include a channel region which is formed usingan oxide semiconductor having a hydrogen concentration of 5×10¹⁹atoms/cm³ or lower.

Another embodiment of the present invention is an analog circuit whichincludes: a first thin film transistor having a first terminal which iselectrically connected to a high power supply potential (also referredto as a high potential side power supply); a second thin film transistorhaving a first terminal which is electrically connected to the highpower supply potential; and a detector between the high power supplypotential and the first terminal of the first thin film transistor. Agate of the first thin film transistor is electrically connected to apoint between the detector and the first terminal of the first thin filmtransistor. A gate of the second thin film transistor is electricallyconnected to the gate of the first thin film transistor. A secondterminal of the first thin film transistor and a second terminal of thesecond thin film transistor are electrically connected to a low powersupply potential (also referred to as a low potential side powersupply). The first thin film transistor and the second thin filmtransistor include a channel region which is formed using an oxidesemiconductor having a hydrogen concentration of 5×10¹⁹ atoms/cm³ orlower.

In this specification, the concentration is measured by secondary ionmass spectrometry (hereinafter also referred to as SIMS). However, themeasurement method is not limited to SIMS when particular description ofanother measurement method is made.

Another embodiment of the present invention is a semiconductor deviceincluding any of the above-described analog circuits.

With an embodiment of the present invention, an analog circuit is formedusing a thin film transistor in which a purified oxide semiconductor isused, whereby a semiconductor device having a high sensitivity indetecting signals and a wide dynamic range can be obtained.

By using the thin film transistor in which a highly-purified oxidesemiconductor is used, a semiconductor device which operates stably andhas low power consumption can be obtained.

BRIEF DESCRIPTION OF DRAWINGS

In the accompanying drawings:

FIG. 1 shows a circuit structure of a semiconductor device;

FIG. 2 shows a cross-sectional structure of a semiconductor device;

FIGS. 3A and 3B show a top-surface structure and a cross-sectionalstructure, respectively, of a semiconductor device;

FIGS. 4A to 4E show a manufacturing process of a semiconductor device;

FIGS. 5A and 5B show a top-surface structure and a cross-sectionalstructure, respectively, of a semiconductor device;

FIGS. 6A to 6E show a manufacturing process of a semiconductor device;

FIGS. 7A and 7B each show a cross-sectional structure of a semiconductordevice;

FIGS. 8A to 8E show a manufacturing process of a semiconductor device;

FIGS. 9A to 9D show a manufacturing process of a semiconductor device;

FIGS. 10A to 10D show a manufacturing process of a semiconductor device;

FIG. 11 shows a cross-sectional structure of a semiconductor device;

FIGS. 12A to 12C each show a semiconductor device;

FIG. 13 shows an equivalent circuit of a pixel in a semiconductordevice;

FIG. 14 shows an equivalent circuit of a pixel in a semiconductordevice;

FIGS. 15A to 15C each show a cross-sectional structure of asemiconductor device;

FIGS. 16A and 16B show a semiconductor device;

FIG. 17 shows a semiconductor device;

FIGS. 18A and 18B each show a semiconductor device;

FIGS. 19A and 19B each show a semiconductor device;

FIG. 20 shows a semiconductor device;

FIG. 21 shows a semiconductor device;

FIG. 22 is a longitudinal section view of an inverted staggered thinfilm transistor in which an oxide semiconductor is used;

FIGS. 23A and 23B are energy band diagrams (schematic diagrams) alongthe section A-A′ of FIG. 22;

FIG. 24A shows a state in which a positive potential (+VG) is applied toa gate (G1), and FIG. 24B shows a state in which a negative potential(−VG) is applied to the gate (G1); and

FIG. 25 shows a relation between the vacuum level and the work functionof a metal (ϕM), and between the vacuum level and the electron affinityof an oxide semiconductor (X).

BEST MODE FOR CARRYING OUT THE INVENTION

Embodiments of the present invention will be described with reference tothe drawings. However, the present invention is not limited to thefollowing description, and it will be easily understood by those skilledin the art that various changes and modifications can be made in modesand details without departing from the spirit and scope of the presentinvention. Therefore, the present invention should not be construed asbeing limited to the description of the following embodiments.

Note that functions of the source and the drain may be switched in thecase where transistors of different polarities are employed or in thecase where the direction of a current flow changes in a circuitoperation. Therefore, the terms “source” and “drain” can be used todenote the drain and the source, respectively, in this specification.

Note that since a source terminal and a drain terminal of a transistorswitch depending on the structure, operating conditions, and the like ofthe transistor, it is difficult to define which is a source terminal ora drain terminal. Therefore, one of a source terminal and a drainterminal is referred to as a first terminal and the other is referred toas a second terminal for distinction in this specification.

Note that the size, the thickness of a layer, or a region of eachstructure illustrated in drawings or the like of embodiments isexaggerated for simplicity in some cases. Therefore, embodiments of thepresent invention are not limited to such scales. Further, in thisspecification, ordinal numbers such as “first”, “second”, and “third”are used in order to avoid confusion among components, and the terms donot limit the components numerically.

Embodiment 1

A thin film transistor disclosed in this specification in which apurified oxide semiconductor is used can be applied to an analogcircuit. As a typical example of the analog circuit, a current mirrorcircuit can be given, for example. In the case of using a current mirrorcircuit as a current amplifier circuit, the dynamic range of the currentamplifier circuit can be increased, and particularly the sensitivity toa small electric current can be improved.

In this embodiment, a photodetector will be described with reference toFIG. 1 as an example of applying the thin film transistor in which apurified oxide semiconductor is used to a current mirror circuit.

A photodetector 1300 illustrated in FIG. 1 includes a detector 1301, anamplifier circuit 1302, a power supply terminal 1311 to which a highpower supply potential VDD is supplied, a power supply terminal 1312 towhich a low power supply potential VSS is supplied, and a protectioncircuit 1320. In the photodetector 1300, a potential of the power supplyterminal 1312 can be a ground potential GND.

The protection circuit 1320 includes a diode 1321. The diode 1321 isprovided between the power supply terminal 1311 and the power supplyterminal 1312, a cathode of the diode 1321 is electrically connected tothe power supply terminal 1311, and an anode of the diode 1321 iselectrically connected to the power supply terminal 1312. In the casewhere an overvoltage (a surge) is applied to the power supply terminal1311 and/or the power supply terminal 1312 because of ESD or the like,the power supply terminal 1311 and the power supply terminal 1312 areshort-circuited by the diode 1321, so that overvoltage can be preventedfrom being applied to the amplifier circuit 1302 and the detector 1301.

As the diode 1321, a thin film transistor whose gate terminal and drainterminal are connected to each other and which has similarcharacteristics as a diode can be used. A thin film transistor using apurified oxide semiconductor disclosed in this specification has afeature of an extremely small off-current value. By forming a diode withthe use of the thin film transistor using a purified oxide semiconductordisclosed in this specification, leak current of the diode when beingapplied with a reverse bias can be extremely small. A plurality ofdiodes formed using such thin film transistors may be connected inseries or in parallel.

A photoelectric conversion element which converts received light intoelectric signals is used as the detector 1301. As the photoelectricconversion element, a photodiode, a phototransistor, or the like can beused. The amplifier circuit 1302 is a circuit for amplifying an outputcurrent of the detector 1301. Here, the amplifier circuit 1302 is formedusing a current mirror circuit. The current mirror circuit includes onetransistor 1305 and a plurality of transistors 1306 connected inparallel to each other. Thin film transistors in which a purified oxidesemiconductor is used are employed as the transistors 1305 and 1306.

The transistor 1305 is a reference transistor for detecting an outputcurrent of the detector 1301, and the transistors 1306 are mirrortransistors which operate based on the current detected by thetransistor 1305.

Current flow between the power supply terminal 1311 and the power supplyterminal 1312 can be adjusted by the number of transistors 1306. Forexample, in order to amplify the current flowing between the powersupply terminal 1311 and the power supply terminal 1312 to an amount 100times as large as the output current of the detector 1301 in the casewhere the transistor 1305 and the transistors 1306 are formed usingtransistors having the same current-voltage characteristics, ninety-ninetransistors 1306 are connected in parallel for one transistor 1305. Withthis structure, the amplifier circuit 1302 formed using the currentmirror circuit can become an amplifier circuit having an amplificationfactor of 100, so that the output current from the detector 1301 can beamplified by 100 times and detected.

The transistors 1306 may have the same structure as the transistor 1305except the channel width that is longer than that of the transistor1305, and the above-described amplifier circuit 1302 can be formed usingthe transistor 1305 and the transistor 1306 having such a structure. Forexample, in the case where twenty transistors 1306 each having a channelwidth that is 4.95 times as long as that of the transistor 1305 are usedand connected in parallel for one transistor 1305, the amplificationfactor of the amplifier circuit 1302 can be 100.

Further, the amplifier circuit 1302 having an amplification factor of100 can also be formed in the case where the transistor 1306 has thesame structure as the transistor 1305 except the channel width that is99 times as long as that of the transistor 1305 and one transistor 1306is connected for one transistor 1305. This structure has an advantage ofsimple circuit structure but increases the possibility of significantlyimpairing the whole function of the amplifier circuit 1302 when thefunction of the transistor 1306 is impaired.

Therefore, for the increase in redundancy, it is preferable that theamplifier circuit 1302 have a structure in which the plurality oftransistors 1306 is connected in parallel. With the structure in whichthe plurality of transistors 1306 is connected in parallel, even if thefunction of part of the plurality of transistors 1306 is impaired, theinfluence on the amplifier circuit 1302 can be suppressed and thus thehighly-reliable amplifier circuit 1302 that can operate stably can beobtained. For example, in the case where ten transistors 1306 areconnected in parallel, even if the function of one of the transistors1306 is impaired, the influence therefrom is supposed to be one tenth.

Further, when the plurality of transistors 1306 is connected inparallel, characteristic variation of the transistors 1306 can bereduced and the amplifier circuit 1302 can operate stably with highreliability.

The amplifier circuit 1302 can have high reliability when the number oftransistors 1306 connected in parallel is two or more, preferably fiveor more. Thus, a photodetector using a highly-reliable current mirrorcircuit can be manufactured.

In the case where the thin film transistors used in the amplifiercircuit 1302 have high off-state current, the signal-to-noise ratiobecomes low at the time of detecting a small amount of light. That is,the off-state current becomes an unignorable amount relative to theoutput current from the detector 1301, so that the output current fromthe detector 1301 cannot be obtained precisely.

In conventional thin film transistors in which amorphous silicon orpolycrystalline silicon is used, the off-state current can be made smallto some extent by reducing the channel width or increasing the channellength; however a reduction in an on current is a problem at this time.Thus, it has been difficult to balance the detection of a small amountof light and detection of a large amount of light and to obtain a widedynamic range.

In a thin film transistor disclosed in this specification in which apurified oxide semiconductor is used, even if the channel width isincreased, the off-state current can be much smaller than that ofconventional thin film transistors. Accordingly, a current mirrorcircuit which has favorable sensitivity to a small amount of current andhas a wide dynamic range can be manufactured. In other words, aphotodetector having a wide dynamic range can be manufactured.

The photodetector in which a photoelectric conversion element is used asthe detector 1301 has been described in this embodiment, any of theother detectors can be employed as the detector 1301. For example, atemperature sensor can be employed as the detector 1301 to form atemperature detection device. Alternatively, an audio sensor can beemployed as the detector 1301, so that not only an audio detectiondevice but also an audio amplifier can be formed.

A current mirror circuit using a thin film transistor disclosed in thisspecification in which a purified oxide semiconductor is used is notlimited to the photodetector described in this embodiment and can beapplied to another semiconductor device.

Embodiment 2

In this embodiment, an example of a stacked structure of thephotodetector 1300 described in Embodiment 1 will be described. Anexample in which a photodiode is used as the detector 1301 will bedescribed in this embodiment. FIG. 2 is a cross-sectional viewillustrating part of the photodetector 1300.

FIG. 2 is a cross-sectional view illustrating the detector 1301 and thetransistor 1305 in the photosensor. The detector 1301 functioning as asensor and the transistor 1305 are provided over a substrate 601. Asubstrate 613 is provided over the detector 1301 and the transistor 1305with an adhesive layer 608 interposed therebetween.

A substrate used as the substrate 601 needs to have a light-transmittingproperty and also have such heat resistance as to withstand heattreatment in the manufacturing process. For example, a glass substratemade of aluminosilicate glass, barium borosilicate glass,aluminoborosilicate glass, or the like can be used. Alternatively, aplastic substrate or the like can be used as appropriate.

An insulating layer 631, a protective insulating layer 632, aninterlayer insulating layer 633, and an interlayer insulating layer 634are provided over the transistor 1305. The detector 1301 is providedover the interlayer insulating layer 633 and has a structure in which afirst semiconductor layer 606 a, a second semiconductor layer 606 b, anda third semiconductor layer 606 c are stacked from the interlayerinsulating layer 633 side. The first semiconductor layer 606 a iselectrically connected to an electrode layer 641 which is provided overthe interlayer insulating layer 633, and the third semiconductor layer606 c is electrically connected to an electrode layer 642 which isprovided over the interlayer insulating layer 634.

The electrode layer 641 is electrically connected to a conductive layer643 which is formed in the interlayer insulating layer 634, and theelectrode layer 642 is electrically connected to a gate electrode layer645 through an electrode layer 644. The gate electrode layer 645 iselectrically connected to a gate electrode layer of the transistor 1305.That is, the detector 1301 is electrically connected to the transistor1305.

Here, a pin photodiode in which a semiconductor layer having a p-typeconductivity as the first semiconductor layer 606 a, a high-resistancesemiconductor layer (I-type semiconductor layer) as the secondsemiconductor layer 606 b, and a semiconductor layer having an n-typeconductivity as the third semiconductor layer 606 c are stacked isillustrated as an example.

The first semiconductor layer 606 a is a p-type semiconductor layer andcan be formed using an amorphous silicon film containing an impurityelement imparting a p-type conductivity. The first semiconductor layer606 a is formed by a plasma CVD method with use of a semiconductorsource gas containing an impurity element belonging to Group 13 (such asboron (B)). As the semiconductor source gas, silane (SiH₄) may be used.Alternatively, Si₂H₆, SiH₂Cl₂, SiHCl₃, SiCl₄, SiF₄, or the like may beused. Further alternatively, an amorphous silicon film which does notcontain an impurity element may be formed, and then, an impurity elementmay be introduced to the amorphous silicon film with use of a diffusionmethod, an ion doping method or an ion implantation method. Heating orthe like may be conducted after introducing the impurity element by anion implantation method or the like in order to diffuse the impurityelement. In this case, as a method of forming the amorphous siliconfilm, an LPCVD method, a vapor deposition method, a sputtering method,or the like may be used. The first semiconductor layer 606 a ispreferably formed to have a thickness greater than or equal to 10 nm andless than or equal to 50 nm.

The second semiconductor layer 606 b is an I-type semiconductor layer(intrinsic semiconductor layer) and is formed using an amorphous siliconfilm. As for formation of the second semiconductor layer 606 b, anamorphous silicon film is formed with use of a semiconductor source gasby a plasma CVD method. As the semiconductor source gas, silane (SiH₄)may be used. Alternatively, Si₂H₆, SiH₂Cl₂, SiHCl₃, SiCl₄, SiF₄, or thelike may be used. The second semiconductor layer 606 b may bealternatively formed by an LPCVD method, a vapor deposition method, asputtering method, or the like. The second semiconductor layer 606 b ispreferably formed to have a thickness greater than or equal to 200 nmand less than or equal to 1000 nm. Ideally, an intrinsic semiconductorlayer refers to a semiconductor layer which does not contain an impurityand whose Fermi level is positioned substantially in the center of aforbidden band; however, the second semiconductor layer 606 b may beformed using a semiconductor into which an impurity serving as a donor(e.g., phosphorus (P) or the like) or an impurity serving as an acceptor(e.g., boron (B) or the like) is added in order that the Fermi level ispositioned substantially in the center of the forbidden band.

The third semiconductor layer 606 c is an n-type semiconductor layer andis formed using an amorphous silicon film containing an impurity elementimparting an n-type conductivity. The third semiconductor layer 606 c isformed by a plasma CVD method with use of a semiconductor source gascontaining an impurity element belonging to Group 15 (e.g., phosphorus(P)). As the semiconductor source gas, silane (SiH₄) may be used.Alternatively, Si₂H₆, SiH₂Cl₂, SiHCl₃, SiCl₄, SiF₄, or the like may beused. Further alternatively, an amorphous silicon film which does notcontain an impurity element may be formed, and then, an impurity elementmay be introduced to the amorphous silicon film with use of a diffusionmethod, an ion doping method or an ion implantation method.

Heating or the like may be conducted after introducing the impurityelement by an ion implantation method or the like in order to diffusethe impurity element In this case, as a method of forming the amorphoussilicon film, an LPCVD method, a vapor deposition method, a sputteringmethod, or the like may be used. The third semiconductor layer 606 c ispreferably formed to have a thickness greater than or equal to 20 nm andless than or equal to 200 nm.

The first semiconductor layer 606 a, the second semiconductor layer 606b, and the third semiconductor layer 606 c are not necessarily formedusing an amorphous semiconductor, and they may be formed using apolycrystalline semiconductor, a microcrystalline semiconductor, or asemiamorphous semiconductor (an SAS).

Considering Gibbs free energy, the microcrystalline semiconductor is ina metastable state that is intermediate between an amorphous state and asingle crystal state. That is, the microcrystalline semiconductor is ina third state that is stable in terms of free energy, and hasshort-range order and lattice distortion. Columnar or needle-likecrystals grow in the direction of the normal to the surface of thesubstrate. The Raman spectrum of microcrystalline silicon, which is atypical example of a microcrystalline semiconductor, is shifted to alower wavenumber side than 520 cm⁻¹ that represents single crystalsilicon. In other words, the Raman spectrum of microcrystalline siliconhas a peak between 520 cm⁻¹ that represents single crystal silicon and480 cm⁻¹ that represents amorphous silicon. Furthermore, themicrocrystalline semiconductor film contains 1 atomic % or more ofhydrogen or halogen to terminate dangling bonds. The microcrystallinesemiconductor film may further contain a rare gas element such ashelium, argon, krypton, or neon to further promote lattice distortion,whereby a favorable microcrystalline semiconductor film with improvedstability can be obtained.

This microcrystalline semiconductor film can be formed by ahigh-frequency plasma CVD method with a frequency of several tens ofmegahertz to several hundreds of megahertz, or a microwave plasma CVDapparatus with a frequency of 1 GHz or more. Typically, themicrocrystalline semiconductor film can be formed using silicon hydridesuch as SiH₄, Si₂H₆, SiH₂Cl₂, or SiHCl₃, or silicon halide such as SiCl₄or SiF₄, which is diluted with hydrogen. Furthermore, themicrocrystalline semiconductor film can be formed with a gas containingsilicon hydride and hydrogen which is diluted by one or more kinds ofrare gas elements selected from helium, argon, krypton, and neon.

In the dilution of silicon hydride, the flow rate ratio of hydrogen tosilicon hydride is set to 5:1 to 200:1, preferably, 50:1 to 150:1, andmore preferably, 100:1. Further, a carbide gas such as CH₄ or C₂H₆, agermanium gas such as GeH₄ or GeF₄, F₂, or the like may be mixed intothe gas containing silicon.

In addition, since the mobility of holes generated by the photoelectriceffect is lower than that of electrons, a pin photodiode has bettercharacteristics when a surface on the p-type semiconductor layer side isused as a light-receiving plane. Here, an example where light 622 whichthe detector 1301 receives from a surface of the substrate 601, overwhich a pin photodiode is formed, is converted into electric signalswill be described. Further, light from the semiconductor layer having aconductivity type opposite from that of the semiconductor layer on thelight-receiving plane is disturbance light; therefore, the electrodelayer is preferably formed using a light-blocking conductive film. Notethat a surface on the n-type semiconductor layer side can alternativelybe used as the light-receiving plane.

As the substrate 613, a substrate similar to the substrate 601 can beused. Since the substrate 613 is positioned opposite from thelight-receiving plane, a light-blocking substrate such as a metalsubstrate of aluminum, stainless-steel, or the like or a semiconductorsubstrate of silicon or the like can be used as the substrate 613.

The insulating layer 631, the protective insulating layer 632, theinterlayer insulating layer 633, and the interlayer insulating layer 634can be formed using an insulating material by a sputtering method, aspin coating method, a dipping method, spray coating, a dropletdischarge method (e.g., an ink-jet method, screen printing, or offsetprinting), roll coating, curtain coating, knife coating, or the likedepending on the material.

As the insulating layer 631, a single layer or a stacked layer of anoxide insulating layer such as a silicon oxide layer, a siliconoxynitride layer, an aluminum oxide layer, an aluminum oxynitride layer,or the like can be used.

As an inorganic insulating material of the protective insulating layer632, a single layer or a stacked layer of a nitride insulating layersuch as a silicon nitride layer, a silicon nitride oxide layer, analuminum nitride layer, an aluminum nitride oxide layer, or the like canbe used. High-density plasma CVD with use of microwaves (2.45 GHz) ispreferably employed since formation of a dense and high-qualityinsulating layer having high withstand voltage is possible.

For reduction of the surface roughness, an insulating film functioningas a planarization insulating film is preferably used as the interlayerinsulating layers 633 and 634. The interlayer insulating layers 633 and634 can be formed using an organic material having heat resistance suchas an acrylic resin, polyimide, a benzocyclobutene-based resin,polyamide, or an epoxy resin. As an alternative to such organicmaterials, it is possible to use a single layer or stacked layers of alow-dielectric constant material (a low-k material), a siloxane-basedresin, phosphosilicate glass (PSG), borophosphosilicate glass (BPSG), orthe like.

By the detector 1301 detecting incident light, information on an objectcan be read. Note that a light source such as a backlight can be used atthe time of reading information on an object.

A transistor described as an example in the above embodiment can be usedas the transistor 1305. A transistor including an oxide layer purifiedby intentionally eliminating an impurity such as hydrogen, moisture,hydroxyl, or hydride (also referred to as a hydrogen compound) from anoxide semiconductor layer has a suppressed variation in electriccharacteristics and is electrically stable. Therefore, a semiconductordevice with high reliability can be provided.

This embodiment can be implemented in appropriate combination with anystructure described in the other embodiments.

Embodiment 3

In this embodiment, an example of a thin film transistor which isincluded in the analog circuit described in Embodiment 1 will bedescribed.

An embodiment of the thin film transistor of this embodiment and amanufacturing method thereof will be described with reference to FIGS.3A and 3B and FIGS. 4A to 4E.

An example of a top-surface structure and a cross-sectional structure ofthe thin film transistor are illustrated in FIGS. 3A and 3B. A thin filmtransistor 410 illustrated in FIGS. 3A and 3B is a thin film transistorhaving a top-gate structure.

FIG. 3A is a top view of the thin film transistor 410 having a top-gatestructure, and FIG. 3B is a cross-sectional structure taken along lineC1-C2 of FIG. 3A.

The thin film transistor 410 includes, over a substrate 400 having aninsulating surface, an insulating layer 407, an oxide semiconductorlayer 412, a source or drain electrode layer 415 a, a source or drainelectrode layer 415 b, a gate insulating layer 402, and a gate electrodelayer 411. A wiring layer 414 a and a wiring layer 414 b are provided incontact with and electrically connected to the source or drain electrodelayer 415 a and the source or drain electrode layer 415 b, respectively.

Although the thin film transistor 410 is described as a single-gate thinfilm transistor, a multi-gate thin film transistor including a pluralityof channel regions can be formed when needed.

A process for manufacturing the thin film transistor 410 over thesubstrate 400 will be described below with reference to FIGS. 4A to 4E.

Although there is no particular limitation on a substrate which can beused as the substrate 400 having an insulating surface, the substrateneeds to have at least heat resistance high enough to withstand heattreatment to be performed later. As the substrate 400 having aninsulating surface, a glass substrate formed of barium borosilicateglass, aluminoborosilicate glass, or the like can be used.

In the case where a glass substrate is used and the temperature of theheat treatment to be performed later is high, a glass substrate whosestrain point is greater than or equal to 730° C. is preferably used. Asthe glass substrate, a substrate of a glass material such asaluminosilicate glass, aluminoborosilicate glass, or barium borosilicateglass is used, for example. Note that by containing a larger amount ofbarium oxide (BaO) than boron oxide (B₂O₃), a glass substrate that isheat-resistant and of more practical use can be obtained. Therefore, aglass substrate containing BaO and B₂O₃ so that the amount of BaO islarger than that of B₂O₃ is preferably used.

Note that a substrate formed of an insulator such as a ceramicsubstrate, a quartz substrate, or a sapphire substrate may be usedinstead of the above glass substrate. Alternatively, crystallized glassor the like can be used. Further alternatively, a plastic substrate orthe like can be used as appropriate.

The insulating layer 407 serving as a base film is formed over thesubstrate 400 having an insulating surface. The insulating layer 407that is in contact with the oxide semiconductor layer 412 is preferablyformed using an oxide insulating layer such as a silicon oxide layer, asilicon oxynitride layer, an aluminum oxide layer, an aluminumoxynitride layer, or the like.

As a formation method of the insulating layer 407, a plasma CVD method,a sputtering method, or the like can be used. In order to prevent theinsulating layer 407 from containing a large amount of hydrogen, theinsulating layer 407 is preferably formed by a sputtering method.

In this embodiment, a silicon oxide layer is formed by a sputteringmethod as the insulating layer 407. The silicon oxide layer is formed asthe insulating layer 407 over the substrate 400 in such a manner thatthe substrate 400 is transferred to a treatment chamber, a sputteringgas containing high-purity oxygen from which hydrogen and moisture havebeen removed is introduced, and a silicon semiconductor target is used.Further, the substrate 400 may be kept at room temperature or may beheated.

For example, the silicon oxide layer is formed by a RF sputtering methodin such conditions that quartz (preferably, synthetic quartz) is used,the substrate temperature is 108° C., the distance between the substrateand the target (T-S distance) is 60 mm, the pressure is 0.4 Pa, thehigh-frequency power is 1.5 kW, and an atmosphere of oxygen and argon(the flow ratio of an oxygen flow rate of 25 sccm to an argon flow rateof 25 sccm=1:1) is used. The thickness is 100 nm. Instead of quartz(preferably, synthetic quartz), a silicon target can be used as thetarget for forming the silicon oxide layer. Oxygen or a mixed gas ofoxygen and argon is used as the sputtering gas.

In this case, it is preferable to remove residual moisture in thetreatment chamber during the film formation of the insulating layer 407,in order to prevent hydroxyl or moisture from being contained in theinsulating layer 407.

In order to remove residual moisture from the treatment chamber, anadsorption-type vacuum pump is preferably used. For example, a cryopump,an ion pump, or a titanium sublimation pump is preferably used. As anexhaustion unit, a turbo pump to which a cold trap is added may be used.From the chamber in which exhaustion is performed with the use of acryopump, a hydrogen atom, a compound including a hydrogen atom such aswater (H₂O), or the like, for example, is exhausted. Accordingly, theconcentration of an impurity included in the insulating layer 407 formedin the film formation chamber can be reduced.

As a sputtering gas used in formation of the insulating layer 407, ahigh-purity gas in which the concentration of an impurity such ashydrogen, water, hydroxyl, or hydride is reduced to approximately theppm level or the ppb level is preferably used.

Examples of a sputtering method include an RF sputtering method in whicha high-frequency power source is used as a sputtering power source, a DCsputtering method in which a direct-current power source is used, and apulsed DC sputtering method in which a bias is applied in a pulsedmanner. An RF sputtering method is mainly used in the case of forming aninsulating film, and a DC sputtering method is mainly used in the caseof forming a metal film.

In addition, there is also a multi-source sputtering apparatus in whicha plurality of targets of different materials can be set. With themulti-source sputtering apparatus, films of different materials can bedeposited to be stacked in one chamber, and films of plural kinds ofmaterials can be deposited by electric discharge at the same time in onechamber.

In addition, there are also a sputtering apparatus provided with amagnet system inside the chamber and used for a magnetron sputteringmethod, and a sputtering apparatus used for an ECR sputtering method inwhich plasma generated with the use of microwaves is used without usingglow discharge.

In addition, as a film formation method using a sputtering method, thereare also a reactive sputtering method in which a target substance and asputtering gas component are chemically reacted with each other duringfilm formation to form a thin film of a compound thereof, and a biassputtering method in which voltage is also applied to a substrate duringfilm formation.

The insulating layer 407 may have a stacked structure and, for example,may have a stacked structure in which a nitride insulating layer such asa silicon nitride layer, a silicon nitride oxide layer, an aluminumnitride layer, or an aluminum nitride oxide layer and theabove-described oxide insulating layer are stacked in this order overthe substrate 400.

For example, a silicon nitride layer is formed using a silicon target byintroducing a sputtering gas which contains high-purity nitrogen andfrom which hydrogen and moisture have been removed, to a space betweenthe silicon oxide layer and the substrate. Also in this case, it ispreferable that the silicon nitride layer be formed while residualmoisture in the treatment chamber is removed in a manner similar to thatof the silicon oxide layer.

Also in the case of forming the silicon nitride layer, the substrate maybe heated during the film formation.

In the case where a silicon nitride layer and a silicon oxide layer arestacked as the insulating layer 407, the silicon nitride layer and thesilicon oxide layer can be formed in the same treatment chamber using acommon silicon target. First, a sputtering gas containing nitrogen isintroduced and a silicon nitride layer is formed using a silicon targetplaced inside the treatment chamber, and then the sputtering gas isswitched to a sputtering gas containing oxygen and a silicon oxide layeris formed using the same silicon target. Since the silicon nitride layerand the silicon oxide layer can be formed in succession without exposureto the air, an impurity such as hydrogen or moisture can be preventedfrom being adsorbed on a surface of the silicon nitride layer.

Then, an oxide semiconductor film is formed to a thickness of greaterthan or equal to 2 nm and less than or equal to 200 nm over theinsulating layer 407.

In order that the oxide semiconductor film contains as little hydrogen,hydroxyl, and moisture as possible, it is preferable that an impurityadsorbed on the substrate 400, such as hydrogen or moisture, beeliminated and exhausted by preheating the substrate 400, over which theinsulating layer 407 is formed, in a preheating chamber of a sputteringapparatus, as a pretreatment for film formation. As an exhaustion unitprovided for the preheating chamber, a cryopump is preferably used. Notethat this preheating treatment can be omitted. This preheating may beperformed on the substrate 400 before formation of the gate insulatinglayer 402 to be formed later or may be performed on the substrate 400over which the source or drain electrode layer 415 a and the source ordrain electrode layer 415 b have been formed, in a similar manner.

Note that before the oxide semiconductor film is formed by a sputteringmethod, dust on a surface of the insulating layer 407 is preferablyremoved by reverse sputtering in which an argon gas is introduced andplasma is generated. The reverse sputtering is a method in which,without application of a voltage to a target side, a voltage is appliedto a substrate side with use of a high-frequency power source in anargon atmosphere and plasma is generated in the vicinity of thesubstrate so that a substrate surface is modified. Note that instead ofan argon atmosphere, a nitrogen atmosphere, a helium atmosphere, anoxygen atmosphere, or the like may be used.

The oxide semiconductor film is formed by a sputtering method. As theoxide semiconductor film, any of the following is used: anIn—Ga—Zn—O-based oxide semiconductor film, an In—Sn—Zn—O-based oxidesemiconductor film, an In—Al—Zn—O-based oxide semiconductor film, aSn—Ga—Zn—O-based oxide semiconductor film, an Al—Ga—Zn—O-based oxidesemiconductor film, a Sn—Al—Zn—O-based oxide semiconductor film, anIn—Sn—O-based oxide semiconductor film, an In—Zn—O-based oxidesemiconductor film, a Sn—Zn—O-based oxide semiconductor film, anAl—Zn—O-based oxide semiconductor film, an In—Ga—O-based oxidesemiconductor film, an In—O-based oxide semiconductor film, a Sn—O-basedoxide semiconductor film, and a Zn—O-based oxide semiconductor film. Inthis embodiment, the oxide semiconductor film is formed by a sputteringmethod with use of an In—Ga—Zn—O-based metal oxide target. The oxidesemiconductor film can be formed by a sputtering method in a rare gas(typically, argon) atmosphere, an oxygen atmosphere, or a mixedatmosphere including a rare gas (typically, argon) and oxygen. In thecase of using a sputtering method, a film may be formed using a targetincluding silicon oxide (SiO_(x) (x>0)) at 2 wt % to 10 wt % inclusive.Addition of silicon oxide (SiO_(x) (x>0)) which hinders crystallizationinto the oxide semiconductor layer can suppress crystallization of theoxide semiconductor layer at the time when heat treatment is performedafter formation of the oxide semiconductor layer in the manufacturingprocess. The oxide semiconductor layer is preferably in an amorphousstate; however, the oxide semiconductor layer may be partlycrystallized.

The oxide semiconductor preferably includes In, and further preferablyincludes In and Ga. In order to obtain an I-type (intrinsic) oxidesemiconductor layer, dehydration or dehydrogenation to be describedlater is effective.

As a sputtering gas used in formation of the oxide semiconductor film, ahigh-purity gas in which the concentration of an impurity such ashydrogen, water, hydroxyl, or hydride is reduced to approximately theppm level or the ppb level is preferably used.

As a target for forming the oxide semiconductor film by a sputteringmethod, a metal oxide target containing zinc oxide as its main componentcan be used. As another example of the metal oxide target, a metal oxidetarget including In, Ga, and Zn (In₂O₃:Ga₂O₃:ZnO=1:1:1 in a molar ratio,In:Ga:Zn=1:1:0.5 in an atomic ratio) can be used. As the metal oxidetarget including In, Ga, and Zn, any of targets having the followingcomposition can also be used: In:Ga:Zn=1:1:1 or 1:1:2 in an atomicratio. The filling rate of the metal oxide target is higher than orequal to 90% and lower than or equal to 100%, and preferably higher thanor equal to 95% and lower than or equal to 99.9%. The use of the metaloxide target having a high filling rate makes it possible to form adense oxide semiconductor film.

The oxide semiconductor film is formed over the substrate 400 in such amanner that the substrate is held inside a treatment chamber which iskept in a reduced pressure state, a sputtering gas from which hydrogenand moisture have been removed is introduced while residual moisture inthe treatment chamber is removed, and a metal oxide is used as a target.In order to remove residual moisture from the treatment chamber, anadsorption-type vacuum pump is preferably used. For example, a cryopump,an ion pump, or a titanium sublimation pump is preferably used. As anexhaustion unit, a turbo pump to which a cold trap is added may be used.From the chamber in which exhaustion is performed with the use of acryopump, a hydrogen atom, a compound including a hydrogen atom such aswater (H₂O) (preferably, a compound including a carbon atom as well), orthe like, for example, is exhausted. Accordingly, the concentration ofan impurity included in the oxide semiconductor film formed in the filmformation chamber can be reduced. The substrate may be heated when theoxide semiconductor film is formed.

As an example of film formation conditions, the following conditions areemployed: the substrate temperature is room temperature, the distancebetween the substrate and the target is 60 mm, the pressure is 0.4 Pa,the direct-current (DC) power is 0.5 kW, and an atmosphere of oxygen andargon (at an oxygen flow rate of 15 sccm and at an argon flow rate of 30sccm) is used. A pulse direct current (DC) power supply is preferablebecause powder substances (also referred to as particles or dust)generated in the film formation can be reduced and the film thicknesscan be made uniform. The oxide semiconductor film preferably has athickness greater than or equal to 5 nm and less than or equal to 30 nm.Note that appropriate thickness of the oxide semiconductor film variesdepending on the material; therefore, the thickness may be determined asappropriate depending on the material.

Next, the oxide semiconductor film is processed into the island-shapedoxide semiconductor layer 412 in a first photolithography step (see FIG.4A). Further, a resist mask for forming the island-shaped oxidesemiconductor layer 412 may be formed using an ink-jet method. When theresist mask is formed by an ink-jet method, a photomask is not used;thus, the manufacturing cost can be reduced.

For this etching of the oxide semiconductor film, wet etching, dryetching, or both of them may be employed.

As the etching gas for dry etching, a gas containing chlorine(chlorine-based gas such as chlorine (Cl₂), boron chloride (BCl₃),silicon chloride (SiCl₄), or carbon tetrachloride (CCl₄)) is preferablyused.

Alternatively, a gas containing fluorine (fluorine-based gas such ascarbon tetrafluoride (CF₄), sulfur fluoride (SF₆), nitrogen fluoride(NF₃), or trifluoromethane (CHF₃)); hydrogen bromide (HBr); oxygen (O₂);any of these gases to which a rare gas such as helium (He) or argon (Ar)is added; or the like can be used.

As the dry etching method, a parallel plate RIE (reactive ion etching)method or an ICP (inductively coupled plasma) etching method can beused. In order to etch the layer into a desired shape, the etchingconditions (the amount of electric power applied to a coil-shapedelectrode, the amount of electric power applied to an electrode on asubstrate side, the temperature of the electrode on the substrate side,or the like) are adjusted as appropriate.

As an etchant used for wet etching, a solution obtained by mixingphosphoric acid, acetic acid, and nitric acid, or the like can be used.In addition, ITO07N (produced by KANTO CHEMICAL CO., INC.) may also beused.

After the wet etching, the etchant is removed by cleaning together withthe material which is etched off. Waste liquid of the etchant containingthe removed material may be purified and the material contained in thewaste liquid may be reused. When a material such as indium included inthe oxide semiconductor layer is collected from the waste liquid afterthe etching and reused, the resources can be efficiently used and thecost can be reduced.

The etching conditions (such as an etchant, etching time, andtemperature) are appropriately adjusted depending on the material sothat the material can be etched into a desired shape.

In this embodiment, the oxide semiconductor film is processed into theisland-shaped oxide semiconductor layer 412 by a wet etching methodusing a solution obtained by mixing phosphoric acid, acetic acid, andnitric acid as an etchant.

Next in this embodiment, first heat treatment is performed on the oxidesemiconductor layer 412. The temperature of the first heat treatment ishigher than or equal to 400° C. and lower than or equal to 750° C.,preferably higher than or equal to 400° C. and lower than the strainpoint of the substrate. Here, the substrate is introduced into anelectric furnace which is one of heat treatment apparatuses, heattreatment is performed on the oxide semiconductor layer in a nitrogenatmosphere at 450° C. for one hour, and then, the oxide semiconductorlayer is not exposed to the air so that entry of water and hydrogen intothe oxide semiconductor layer is prevented; thus, the oxidesemiconductor layer is obtained. Dehydration or dehydrogenation of theoxide semiconductor layer 412 can be performed through the first heattreatment.

The heat treatment apparatus is not limited to the electric furnace andmay be the one provided with a device for heating a process object usingheat conduction or heat radiation from a heating element such as aresistance heating element. For example, an RTA (rapid thermalannealing) apparatus such as a GRTA (gas rapid thermal annealing)apparatus or an LRTA (lamp rapid thermal annealing) apparatus can beused. An LRTA apparatus is an apparatus for heating a process object byradiation of light (an electromagnetic wave) emitted from a lamp such asa halogen lamp, a metal halide lamp, a xenon arc lamp, a carbon arclamp, a high-pressure sodium lamp, or a high-pressure mercury lamp. AGRTA apparatus is an apparatus for heat treatment using ahigh-temperature gas. As the gas, an inert gas which does not react witha process object by heat treatment, such as nitrogen or a rare gas suchas argon is used.

For example, as the first heat treatment, GRTA may be performed in thefollowing manner. The substrate is transferred and put in an inert gaswhich has been heated to a high temperature of 650° C. to 700° C.,heated for several minutes, and transferred and taken out of the inertgas which has been heated to a high temperature. GRTA enables ahigh-temperature heat treatment in a short time.

Note that in the first heat treatment, it is preferable that water,hydrogen, and the like be not contained in nitrogen or a rare gas suchas helium, neon, or argon. Alternatively, it is preferable that nitrogenor a rare gas such as helium, neon, or argon introduced into a heattreatment apparatus have a purity of 6N (99.9999%) or more, preferably,7N (99.99999%) or more (that is, the impurity concentration is set to 1ppm or lower, preferably, 0.1 ppm or lower).

By reduction of an impurity in the oxide semiconductor in theabove-described manner, an I-type or substantially I-type oxidesemiconductor (a purified oxide semiconductor) can be obtained.Specifically, hydrogen or OH group in the oxide semiconductor is removedso that hydrogen be included in the oxide semiconductor at lower than orequal to 5×10¹⁹/cm³, preferably lower than or equal to 5×10¹²/cm³, andfurther preferably lower than or equal to 5×10¹⁷/cm³, whereby thecarrier concentration becomes 5×10¹⁴/cm³ or lower, preferably 5×10¹²/cm³or lower. Thus, an I-type or substantially I-type oxide semiconductor (apurified oxide semiconductor) can be obtained.

Further, the oxide semiconductor layer may be crystallized to be amicrocrystalline layer or a polycrystalline layer depending on thecondition of the first heat treatment or a material of the oxidesemiconductor layer. For example, the oxide semiconductor layer may becrystallized to be a microcrystalline oxide semiconductor layer having adegree of crystallization of 90% or more, or 80% or more. The oxidesemiconductor layer may become an amorphous oxide semiconductor layercontaining no crystalline component depending on the condition of thefirst heat treatment or a material of the oxide semiconductor layer. Theoxide semiconductor layer may become an oxide semiconductor layer inwhich a microcrystalline portion (with a grain diameter greater than orequal to 1 nm and less than or equal to 20 nm, typically greater than orequal to 2 nm and less than or equal to 4 nm) is mixed into an amorphousoxide semiconductor.

The first heat treatment may be performed on the oxide semiconductorfilm before being processed into the island-shaped oxide semiconductorlayer, instead of on the island-shaped oxide semiconductor layer. Inthat case, after the first heat treatment, the substrate is taken out ofthe heating apparatus and a photolithography step is performed.

The heat treatment having an effect of dehydrating or dehydrogenatingthe oxide semiconductor layer may be performed at any of the followingtimings: after the oxide semiconductor layer is formed; after a sourceelectrode and a drain electrode are formed over the oxide semiconductorlayer; and after a gate insulating layer is formed over the sourceelectrode and the drain electrode.

Next, a conductive film is formed over the insulating layer 407 and theoxide semiconductor layer 412. The conductive film may be formed by asputtering method or a vacuum evaporation method. As a material of theconductive film, it is possible to use an element selected from aluminum(Al), chromium (Cr), copper (Cu), tantalum (Ta), titanium (Ti),molybdenum (Mo), and tungsten (W), an alloy containing any of theelements, an alloy film combining the elements, or the like.Alternatively, one or more materials selected from manganese (Mn),magnesium (Mg), zirconium (Zr), beryllium (Be), and thorium (Th) may beused. Further, the metal conductive film may have a single-layerstructure or a stacked structure including two or more layers. Forexample, a single-layer structure of an aluminum film including silicon,a two-layer structure in which a titanium film is stacked over analuminum film, a three-layer structure in which a titanium film, analuminum film, and a titanium film are stacked in this order, and thelike can be given. Alternatively, a film, an alloy film, or a nitridefilm which contains aluminum (Al) and one or a plurality of elementsselected from titanium, tantalum, tungsten, molybdenum, chromium,neodymium, and scandium may be used.

A resist mask is formed over the conductive film by a secondphotolithography step. Selective etching is performed, so that a sourceor drain electrode layer 415 a and a source or drain electrode layer 415b are formed. Then, the resist mask is removed (see FIG. 4B). It ispreferable that end portions of the formed source and drain electrodelayers have a tapered shape to improve coverage with a gate insulatinglayer stacked thereover.

In this embodiment, a titanium film having a thickness of 150 nm isformed by a sputtering method as the conductive film for forming thesource or drain electrode layer 415 a and the source or drain electrodelayer 415 b.

Note that each material and etching conditions are adjusted asappropriate so that the insulating layer 407 below the oxidesemiconductor layer 412 is not exposed by removal of the oxidesemiconductor layer 412 at the time of etching the conductive film.

In this embodiment, a Ti film is used as the conductive film, anIn—Ga—Zn—O-based oxide semiconductor is used as the oxide semiconductorlayer 412, and an ammonia hydrogen peroxide solution (a mixture ofammonia, water, and a hydrogen peroxide solution) is used as an etchant.

Note that in the second photolithography step, the oxide semiconductorlayer 412 may be partly etched in some cases, so that an oxidesemiconductor layer having a groove (a depression portion) is formed. Aresist mask for forming the source or drain electrode layer 415 a andthe source or drain electrode layer 415 b may be formed by an ink-jetmethod. When the resist mask is formed by an ink-jet method, a photomaskis not used; thus, the manufacturing cost can be reduced.

For light exposure at the formation of the resist mask in the secondphotolithography step, ultraviolet rays, KrF laser light, or ArF laserlight is used. The channel length L of the thin film transistor to becompleted later is determined by the distance between a lower endportion of the source electrode layer and a lower end portion of thedrain electrode layer, which are adjacent to each other over the oxidesemiconductor layer 412. In the case where the channel length L isshorter than 25 nm and light exposure for forming the resist mask in thesecond photolithography step is performed, extreme ultraviolet rayshaving a wavelength as extremely short as several nanometers to severaltens of nanometers are used. Light exposure using extreme ultravioletrays has high resolution and large depth of focus. Therefore, thechannel length L of the thin film transistor to be completed later canbe set at larger than or equal to 10 nm and less than or equal to 1000nm, whereby circuit operation speed can be increased. In addition, sincethe off-state current is extremely low, lower power consumption can beachieved.

Next, the gate insulating layer 402 is formed over the insulating layer407, the oxide semiconductor layer 412, the source or drain electrodelayer 415 a, and the source or drain electrode layer 415 b (see FIG.4C).

The gate insulating layer 402 can be formed to have a single-layerstructure or a stacked structure of a silicon oxide layer, a siliconnitride layer, a silicon oxynitride layer, a silicon nitride oxidelayer, or an aluminum oxide layer by a plasma CVD method, a sputteringmethod, or the like. In order to prevent the gate insulating layer 402from containing a large amount of hydrogen, the gate insulating layer402 is preferably formed by a sputtering method. In the case of forminga silicon oxide film by a sputtering method, a silicon target or aquartz target is used as a target, and oxygen or a mixed gas of oxygenand argon is used as a sputtering gas.

The gate insulating layer 402 can have a structure in which a siliconoxide layer and a silicon nitride layer are stacked in this order overthe source or drain electrode layer 415 a and source or drain electrodelayer 415 b. For example, the gate insulating layer 402 having athickness of 100 nm may be formed in such a manner that a silicon oxidelayer (SiO_(x) (x>0)) having a thickness of 5 nm to 300 nm inclusive isformed as a first gate insulating layer and a silicon nitride layer(SiN_(y) (y>0)) having a thickness of 50 nm to 200 nm inclusive isstacked over the first gate insulating layer as a second gate insulatinglayer by a sputtering method. In this embodiment, a silicon oxide layerhaving a thickness of 100 nm is formed by an RF sputtering method undera pressure of 0.4 Pa, a high-frequency power of 1.5 kW, and anatmosphere of oxygen and argon (the flow ratio of an oxygen flow rate of25 sccm to an argon flow rate of 25 sccm=1:1).

Next in a third photolithography step, a resist mask is formed, andselective etching is performed to remove part of the gate insulatinglayer 402, so that an opening 421 a and an opening 421 b which reach thesource or drain electrode layer 415 a and the source or drain electrodelayer 415 b respectively are formed (see FIG. 4D).

Then, a conductive film is formed over the gate insulating layer 402 andthe openings 421 a and 421 b, and the gate electrode layer 411 and thewiring layers 414 a and 414 b are formed by a fourth photolithographystep. Note that a resist mask may be formed by an ink-jet method. Whenthe resist mask is formed by an ink-jet method, a photomask is not used;thus, the manufacturing cost can be reduced.

The gate electrode layer 411 and the wiring layers 414 a and 414 b canbe formed to have a single-layer structure or a stacked structure usinga metal material such as molybdenum, titanium, chromium, tantalum,tungsten, aluminum, copper, neodymium, or scandium or an alloy materialwhich contains any of these materials as its main component.

For example, as a two-layer structure of the gate electrode layer 411and the wiring layers 414 a and 414 b, any of the following structuresare preferable: a two-layer structure of an aluminum layer and amolybdenum layer stacked thereover, a two-layer structure of a copperlayer and a molybdenum layer stacked thereover, a two-layer structure ofa copper layer and a titanium nitride layer or a tantalum nitride layerstacked thereover, and a two-layer structure of a titanium nitride layerand a molybdenum layer. As a three-layer structure, a stack of atungsten layer or a tungsten nitride layer, a layer of an alloy ofaluminum and silicon or an alloy of aluminum and titanium, and atitanium nitride layer or a titanium layer is preferable. Note that thegate electrode layer can also be formed using a light-transmittingconductive film. As a light-transmitting conductive film, alight-transmitting conductive oxide or the like can be given.

In this embodiment, the gate electrode layer 411 and the wiring layers414 a and 414 b are formed using a titanium film having a thickness of150 nm by a sputtering method.

Next, second heat treatment (preferably at 200° C. to 400° C., forexample at 250° C. to 350° C.) is performed in an inert gas atmosphereor an oxygen gas atmosphere. In this embodiment, the second heattreatment is performed at 250° C. in a nitrogen atmosphere for one hour.The second heat treatment may be performed after a protective insulatinglayer or a planarization insulating layer is formed over the thin filmtransistor 410.

Furthermore, heat treatment may be performed at 100° C. to 200° C.inclusive for one hour to 30 hours in the air. This heat treatment maybe performed at a fixed heating temperature. Alternatively, thefollowing change in the heating temperature may be conducted pluraltimes repeatedly: the heating temperature is increased from roomtemperature to a temperature of 100° C. to 200° C. and then decreased toroom temperature. This heat treatment may be performed under a reducedpressure. Under a reduced pressure, the heat treatment time can beshortened.

Through the above-described process, the thin film transistor 410including the oxide semiconductor layer 412 in which the concentrationof hydrogen, moisture, hydride, or hydroxide is reduced can be formed(see FIG. 4E). The thin film transistor 410 can be used as the thin filmtransistor included in the analog circuit of Embodiments 1 and 2.

A protective insulating layer or a planarization insulating layer forplanarization may be provided over the thin film transistor 410. Forexample, a protective insulating layer can be formed to have asingle-layer structure or a stacked structure including a silicon oxidelayer, a silicon nitride layer, a silicon oxynitride layer, a siliconnitride oxide layer, or an aluminum oxide layer.

The planarization insulating layer can be formed using an organicmaterial having heat resistance, such as polyimide, an acrylic resin, abenzocyclobutene-based resin, polyamide, or an epoxy resin. Other thansuch organic materials, it is also possible to use a low-dielectricconstant material (a low-k material), a siloxane-based resin, PSG(phosphosilicate glass), BPSG (borophosphosilicate glass), or the like.Note that the planarization insulating layer may be formed by stacking aplurality of insulating films using these materials.

Note that the siloxane-based resin corresponds to a resin including aSi—O—Si bond formed using a siloxane-based material as a startingmaterial. The siloxane-based resin may include as a substituent anorganic group (e.g., an alkyl group or an aryl group) or a fluoro group.In addition, the organic group may include a fluoro group.

There is no particular limitation on the method for forming theplanarization insulating layer. The planarization insulating layer canbe formed, depending on the material, by a method such as a sputteringmethod, an SOG method, a spin coating method, a dipping method, a spraycoating method, or a droplet discharge method (e.g., an ink-jet method,screen printing, or offset printing), or with the use of a tool such asa doctor knife, a roll coater, a curtain coater, or a knife coater.

When the oxide semiconductor film is formed, residual moisture in areaction atmosphere is removed in the above-described manner; thus, theconcentration of hydrogen and hydride in the oxide semiconductor filmcan be reduced. Thus, the oxide semiconductor film can be stabilized.

The oxide semiconductor which becomes I-type or becomes substantiallyI-type (an oxide semiconductor which is purified) due to removal of animpurity is extremely sensitive to an interface state density or aninterface electric charge; therefore, an interface with the gateinsulating film is important. Thus, higher quality is demanded for thegate insulating film (GI) in contact with the purified oxidesemiconductor.

For example, high-density plasma CVD with use of microwaves (2.45 GHz)is preferably employed since formation of a dense and high-qualityinsulating film having high withstand voltage is possible. When thepurified oxide semiconductor and the high-quality gate insulating filmare in close contact with each other, the interface state density can bereduced and favorable interface characteristics can be obtained. If aninsulating film that is favorable as a gate insulating film can beformed, other film formation methods such as a sputtering method and aplasma CVD method can be employed. Alternatively, an insulating filmwhose film quality and interface characteristics with the oxidesemiconductor are improved by heat treatment performed after formationof the insulating film may be used. In any case, any insulating filmthat has a reduced interface state density with the oxide semiconductorand can form a favorable interface as well as having a favorable filmquality as a gate insulating film can be used.

Further, when an oxide semiconductor containing impurities is subjectedto a gate bias-temperature stress test (BT test) at 85° C., at a voltageapplied to the gate of 2×10⁶ V/cm, for 12 hours, a bond between theimpurity and a main component of the oxide semiconductor is cleaved by ahigh electric field (B: bias) and a high temperature (T: temperature),and a generated dangling bond induces drift of threshold voltage(V_(th)). In contrast, the present invention makes it possible to obtaina thin film transistor which is stable to a BT test by removingimpurities in an oxide semiconductor, especially hydrogen, water, andthe like as much as possible to obtain a favorable characteristic of aninterface between the oxide semiconductor and a gate insulating film asdescribed above.

When the above-described thin film transistor is used in the analogcircuit described in Embodiment 1, the analog circuit can have stableelectric characteristics and high reliability.

This embodiment can be implemented in appropriate combination with anyof the other embodiments.

Embodiment 4

In this embodiment, an example of a thin film transistor which isincluded in the analog circuit described in Embodiment 1 will bedescribed. Note that as for the same portions as or portions havingfunctions similar to those in Embodiment 3 and the same steps as orsteps similar to those in Embodiment 3, Embodiment 3 may be referred to,and repetitive description thereof will be omitted. In addition,detailed description of the same portions is not repeated.

In this embodiment, an embodiment of a thin film transistor and amanufacturing method thereof will be described with reference to FIGS.5A and 5B and FIGS. 6A to 6E.

An example of a top-surface structure and a cross-sectional structure ofthe thin film transistor are illustrated in FIGS. 5A and 5B. A thin filmtransistor 460 illustrated in FIGS. 5A and 5B is a thin film transistorhaving a top-gate structure.

FIG. 5A is a plan view of the thin film transistor 460 having a top-gatestructure, and FIG. 5B is a cross-sectional structure taken along lineD1-D2 of FIG. 5A.

The thin film transistor 460 includes, over a substrate 450 having aninsulating surface, an insulating layer 457, source or drain electrodelayers 465 a 1 and 465 a 2, an oxide semiconductor layer 462, a sourceor drain electrode layer 465 b, a wiring layer 468, a gate insulatinglayer 452, and a gate electrode layer 461 (461 a, 461 b). The source ordrain electrode layers 465 a 1 and 465 a 2 are electrically connected tothe wiring layer 464 through the wiring layer 468. In addition, althoughnot illustrated, the source or drain electrode layer 465 b is alsoelectrically connected to a wiring layer through an opening provided inthe gate insulating layer 452.

A process for manufacturing the thin film transistor 460 over thesubstrate 450 will be described below with reference to FIGS. 6A to 6E.

First, the insulating layer 457 serving as a base film is formed overthe substrate 450 having an insulating surface.

In this embodiment, a silicon oxide layer is formed by a sputteringmethod as the insulating layer 457. The silicon oxide layer is formed asthe insulating layer 457 over the substrate 450 in such a manner thatthe substrate 450 is transferred to a treatment chamber, a sputteringgas containing high-purity oxygen from which hydrogen and moisture havebeen removed is introduced, and a silicon target or quartz (syntheticquartz) is used. Oxygen or a mixed gas of oxygen and argon is used as asputtering gas.

For example, the silicon oxide layer is formed by a RF sputtering methodin such conditions that quartz (preferably, synthetic quartz) having apurity of 6N is used, the substrate temperature is 108° C., the distancebetween the substrate and the target (T-S distance) is 60 mm, thepressure is 0.4 Pa, the high-frequency power is 1.5 kW, and anatmosphere of oxygen and argon (the flow ratio of an oxygen flow rate of25 sccm to an argon flow rate of 25 sccm=1:1) is used. The thickness is100 nm. Instead of quartz (preferably, synthetic quartz), a silicontarget can be used as the target for forming the silicon oxide layer.

In this case, it is preferable to remove residual moisture in thetreatment chamber during the film formation of the insulating layer 457,in order to prevent hydroxyl or moisture from being contained in theinsulating layer 457. From the chamber in which exhaustion is performedwith the use of a cryopump, a hydrogen atom, a compound including ahydrogen atom such as water (H₂O), or the like, for example, isexhausted. Accordingly, the concentration of an impurity included in theinsulating layer 457 formed in the film formation chamber can bereduced.

As a sputtering gas used in formation of the insulating layer 457, ahigh-purity gas in which the concentration of an impurity such ashydrogen, water, hydroxyl, or hydride is reduced to approximately theppm level or the ppb level is preferably used.

The insulating layer 457 may have a stacked structure and, for example,may have a stacked structure in which a nitride insulating layer such asa silicon nitride layer, a silicon nitride oxide layer, an aluminumnitride layer, or an aluminum nitride oxide layer and theabove-described oxide insulating layer are stacked in this order overthe substrate 450.

For example, a silicon nitride layer is formed using a silicon target byintroducing a sputtering gas which contains high-purity nitrogen andfrom which hydrogen and moisture have been removed, to a space betweenthe silicon oxide layer and the substrate. Also in this case, it ispreferable that the silicon nitride layer be formed while residualmoisture in the treatment chamber is removed, in a manner similar tothat of the silicon oxide layer.

Next, a conductive film is formed over the insulating layer 457. Aresist mask is formed over the conductive film by a firstphotolithography step. Selective etching is performed, so that thesource or drain electrode layers 465 a 1 and 465 a 2 are formed. Then,the resist mask is removed (see FIG. 6A). The source or drain electrodelayers 465 a 1 and 465 a 2 are separated in the cross-sectional view,they are one continuous film. It is preferable that end portions of theformed source and drain electrode layers have a tapered shape to improvecoverage with a gate insulating layer stacked thereover.

As a material of the source or drain electrode layers 465 a 1 and 465 a2, an element selected from Al, Cr, Cu, Ta, Ti, Mo, and W; an alloycontaining any of the above elements as its component; an alloy filmcontaining these elements in combination; and the like can be given.Alternatively, one or more materials selected from manganese, magnesium,zirconium, beryllium, and thorium may be used. The metal conductive filmmay have a single-layer structure or a stacked structure of two or morelayers. For example, a single-layer structure of an aluminum filmincluding silicon, a two-layer structure in which a titanium film isstacked over an aluminum film, a three-layer structure in which atitanium film, an aluminum film, and a titanium film are stacked in thisorder, and the like can be given. Alternatively, a film, an alloy film,or a nitride film which contains aluminum and one or a plurality ofelements selected from titanium, tantalum, tungsten, molybdenum,chromium, neodymium, and scandium may be used.

In this embodiment, the source or drain electrode layers 465 a 1 and 465a 2 are formed using a titanium film having a thickness of 150 nm by asputtering method.

Next, an oxide semiconductor film having a thickness larger than orequal to 2 nm and less than or equal to 200 nm is formed over theinsulating layer 457 and the source or drain electrode layers 465 a 1and 465 a 2.

Then, the oxide semiconductor film is processed into the island-shapedoxide semiconductor layer 462 by a second photolithography step (seeFIG. 6B). In this embodiment, the oxide semiconductor film is formed bya sputtering method with use of an In—Ga—Zn—O-based metal oxide target.

The oxide semiconductor film is formed over the substrate 450 in such amanner that the substrate is held inside a treatment chamber which iskept in a reduced pressure state, a sputtering gas from which hydrogenand moisture have been removed is introduced while residual moisture inthe treatment chamber is removed, and a metal oxide is used as a target.In order to remove residual moisture from the treatment chamber, anadsorption-type vacuum pump is preferably used. For example, a cryopump,an ion pump, or a titanium sublimation pump is preferably used. As anexhaustion unit, a turbo pump to which a cold trap is added may be used.From the chamber in which exhaustion is performed with the use of acryopump, a hydrogen atom, a compound including a hydrogen atom such aswater (H₂O) (preferably, a compound including a carbon atom as well), orthe like, for example, is exhausted. Accordingly, the concentration ofan impurity included in the oxide semiconductor film formed in the filmformation chamber can be reduced. The substrate may be heated when theoxide semiconductor film is formed.

As a sputtering gas used in formation of the oxide semiconductor film, ahigh-purity gas in which the concentration of an impurity such ashydrogen, water, hydroxyl, or hydride is reduced to approximately theppm level or the ppb level is preferably used.

As an example of film formation conditions, the following conditions areemployed: the substrate temperature is room temperature, the distancebetween the substrate and the target is 60 mm, the pressure is 0.4 Pa,the direct-current (DC) power is 0.5 kW, and an atmosphere of oxygen andargon (at an oxygen flow rate of 15 sccm and at an argon flow rate of 30sccm) is used. A pulse direct current (DC) power supply is preferablebecause powder substances (also referred to as particles or dust)generated in the film formation can be reduced and the film thicknesscan be made uniform. The oxide semiconductor film preferably has athickness greater than or equal to 5 nm and less than or equal to 30 nm.Note that appropriate thickness of the oxide semiconductor film variesdepending on the material; therefore, the thickness may be determined asappropriate depending on the material.

In this embodiment, the oxide semiconductor film is processed into theisland-shaped oxide semiconductor layer 462 by a wet etching methodusing a solution obtained by mixing phosphoric acid, acetic acid, andnitric acid as an etchant.

In this embodiment, first heat treatment is performed on the oxidesemiconductor layer 462. The temperature of the first heat treatment ishigher than or equal to 400° C. and lower than or equal to 750° C.,preferably higher than or equal to 400° C. and lower than the strainpoint of the substrate. Here, the substrate is introduced into anelectric furnace which is one of heat treatment apparatuses, heattreatment is performed on the oxide semiconductor layer in a nitrogenatmosphere at 450° C. for one hour, and then, the oxide semiconductorlayer is not exposed to the air so that entry of water and hydrogen intothe oxide semiconductor layer is prevented; thus, the oxidesemiconductor layer is obtained. Dehydration or dehydrogenation of theoxide semiconductor layer 462 can be performed through the first heattreatment.

The heat treatment apparatus is not limited to the electric furnace andmay be the one provided with a device for heating a process object usingheat conduction or heat radiation from a heating element such as aresistance heating element. For example, an RTA (rapid thermalannealing) apparatus such as a GRTA (gas rapid thermal annealing)apparatus or an LRTA (lamp rapid thermal annealing) apparatus can beused. For example, as the first heat treatment, GRTA may be performed inthe following manner. The substrate is transferred and put in an inertgas which has been heated to a high temperature of 650° C. to 700° C.,heated for several minutes, and transferred and taken out of the inertgas which has been heated to a high temperature. GRTA enables ahigh-temperature heat treatment in a short time.

Note that in the first heat treatment, it is preferable that water,hydrogen, and the like be not contained in nitrogen or a rare gas suchas helium, neon, or argon. Alternatively, it is preferable that nitrogenor a rare gas such as helium, neon, or argon introduced into a heattreatment apparatus have a purity of 6N (99.9999%) or more, preferably,7N (99.99999%) or more (that is, the impurity concentration is set to 1ppm or lower, preferably, 0.1 ppm or lower).

Further, the oxide semiconductor layer may be crystallized to be amicrocrystalline layer or a polycrystalline layer depending on thecondition of the first heat treatment or a material of the oxidesemiconductor layer.

The first heat treatment may be performed on the oxide semiconductorfilm before being processed into the island-shaped oxide semiconductorlayer, instead of on the island-shaped oxide semiconductor layer. Inthat case, after the first heat treatment, the substrate is taken out ofthe heating apparatus and a photolithography step is performed.

The heat treatment having an effect of dehydrating or dehydrogenatingthe oxide semiconductor layer may be performed at any of the followingtimings: after the oxide semiconductor layer is formed; after a sourceelectrode or a drain electrode is stacked over the oxide semiconductorlayer; and after a gate insulating layer is formed over the sourceelectrode and the drain electrode.

Next, a conductive film is formed over the insulating layer 457 and theoxide semiconductor layer 462. A resist mask is formed over theconductive film by a third photolithography step. Selective etching isperformed, so that the source or drain electrode layer 465 b and thewiring layer 468 are formed. Then, the resist mask is removed (see FIG.6C). The source or drain electrode layer 465 b and the wiring layer 468may be formed using a material and a process which are similar to thoseof the source or drain electrode layers 465 a 1 and 465 a 2.

In this embodiment, the source or drain electrode layer 465 b and thewiring layer 468 are formed using a titanium film having a thickness of150 nm by a sputtering method. Since an example where the source ordrain electrode layers 465 a 1 and 465 a 2 and the source or drainelectrode layer 465 b are both formed using a titanium film is describedin this embodiment, the etching selectivity of the source or drainelectrode layer 465 b with respect to the source or drain electrodelayers 465 a 1 and 465 a 2 is not high. Therefore, the wiring layer 468is provided over a part of the source or drain electrode layer 465 a 2,which is not covered with the oxide semiconductor layer 462, in orderthat the source or drain electrode layers 465 a 1 and 465 a 2 are notetched at the time of etching for forming the source or drain electrodelayer 465 b. In the case where the source or drain electrode layers 465a 1 and 465 a 2 and the source or drain electrode layer 465 b are formedusing different materials between which etching selectivity is high inthe etching step, the wiring layer 468 which protects the source ordrain electrode layer 465 a 2 at the time of etching need notnecessarily be formed.

Note that each material and etching conditions are adjusted asappropriate so that the oxide semiconductor layer 462 is not removed byetching of the conductive film.

In this embodiment, a Ti film is used as the conductive film, anIn—Ga—Zn—O-based oxide semiconductor is used as the oxide semiconductorlayer 462, and an ammonia hydrogen peroxide solution (a mixture ofammonia, water, and a hydrogen peroxide solution) is used as an etchant.

Note that in the third photolithography step, the oxide semiconductorlayer 462 may be partly etched in some cases, so that an oxidesemiconductor layer having a groove (a depression portion) is formed. Aresist mask for forming the source or drain electrode layer 465 b andthe wiring layer 468 may be formed by an ink-jet method. When the resistmask is formed by an ink-jet method, a photomask is not used; thus, themanufacturing cost can be reduced.

Next, the gate insulating layer 452 is formed over the insulating layer457, the oxide semiconductor layer 462, the source or drain electrodelayers 465 a 1 and 465 a 2, and the source or drain electrode layer 465b.

The gate insulating layer 452 can be formed to have a single-layerstructure or a stacked structure of a silicon oxide layer, a siliconnitride layer, a silicon oxynitride layer, a silicon nitride oxidelayer, or an aluminum oxide layer by a plasma CVD method, a sputteringmethod, or the like. In order to prevent the gate insulating layer 452from containing a large amount of hydrogen, the gate insulating layer452 is preferably formed by a sputtering method. In the case of forminga silicon oxide film by a sputtering method, a silicon target or aquartz target is used as a target, and oxygen or a mixed gas of oxygenand argon is used as a sputtering gas.

The gate insulating layer 452 can have a structure in which a siliconoxide layer and a silicon nitride layer are stacked from the source ordrain electrode layers 465 a 1 and 465 a 2 and source or drain electrodelayer 465 b side. In this embodiment, a silicon oxide layer having athickness of 100 nm is formed by an RF sputtering method under apressure of 0.4 Pa, a high-frequency power of 1.5 kW, and an atmosphereof oxygen and argon (the flow ratio of an oxygen flow rate of 25 sccm toan argon flow rate of 25 sccm=1:1).

Next in a fourth photolithography step, a resist mask is formed, andselective etching is performed to remove part of the gate insulatinglayer 452, so that an opening 423 which reaches the wiring layer 468 isformed (see FIG. 6D). Although not illustrated, an opening reaching thesource or drain electrode layer 465 b may be formed at the time offorming the opening 423. In this embodiment, after an interlayerinsulating layer is further stacked, the opening reaching the source ordrain electrode layer 465 b is formed, and then a wiring layer forelectric connection is formed in the opening.

Then, a conductive film is formed over the gate insulating layer 452 andthe opening 423, and the gate electrode layer 461 (461 a, 461 b) and thewiring layer 464 are formed by a fifth photolithography step. Note thata resist mask may be formed by an ink-jet method. When the resist maskis formed by an ink-jet method, a photomask is not used; thus, themanufacturing cost can be reduced.

The gate electrode layer 461 (461 a, 461 b) and the wiring layer 464 canbe formed to have a single-layer structure or a stacked structure usinga metal material such as molybdenum, titanium, chromium, tantalum,tungsten, aluminum, copper, neodymium, or scandium or an alloy materialwhich contains any of these materials as its main component.

In this embodiment, the gate electrode layer 461 (461 a, 461 b) and thewiring layer 464 are formed using a titanium film having a thickness of150 nm by a sputtering method.

Next, second heat treatment (preferably at 200° C. to 400° C., forexample at 250° C. to 350° C.) is performed in an inert gas atmosphereor an oxygen gas atmosphere. In this embodiment, the second heattreatment is performed at 250° C. in a nitrogen atmosphere for one hour.The second heat treatment may be performed after a protective insulatinglayer or a planarization insulating layer is formed over the thin filmtransistor 410.

Furthermore, heat treatment may be performed at 100° C. to 200° C.inclusive for one hour to 30 hours in the air. This heat treatment maybe performed at a fixed heating temperature. Alternatively, thefollowing change in the heating temperature may be conducted pluraltimes repeatedly: the heating temperature is increased from roomtemperature to a temperature of 100° C. to 200° C. and then decreased toroom temperature. This heat treatment may be performed under a reducedpressure. Under a reduced pressure, the heat treatment time can beshortened.

Through the above-described process, the thin film transistor 460including the oxide semiconductor layer 462 in which the concentrationof hydrogen, moisture, hydride, or hydroxide is reduced can be formed(see FIG. 6E).

A protective insulating layer or a planarization insulating layer forplanarization may be provided over the thin film transistor 460.Although not illustrated, an opening reaching the source or drainelectrode layer 465 b is formed in the gate insulating layer 452 and theprotective insulating layer or the planarization insulating layer, and awiring layer electrically connected to the source or drain electrodelayer 465 b is formed in the opening.

When the oxide semiconductor film is formed, residual moisture in areaction atmosphere is removed in the above-described manner; thus, theconcentration of hydrogen and hydride in the oxide semiconductor filmcan be reduced. Thus, the oxide semiconductor film can be stabilized.

When the above-described thin film transistor is used in the analogcircuit described in Embodiment 1, the analog circuit can have stableelectric characteristics and high reliability.

This embodiment can be implemented in appropriate combination with anyof the other embodiments.

Embodiment 5

In this embodiment, an example of a thin film transistor included in theanalog circuit described in Embodiment 1 will be described. Note that asfor portions that are the same as those in the other embodiments andportions and steps that are similar to those in the other embodiments,descriptions of the other embodiments can be referred to, anddescriptions thereof are not repeated. In addition, detailed descriptionof the same parts is omitted.

Thin film transistors of this embodiment will be described withreference to FIGS. 7A and 7B.

FIGS. 7A and 7B illustrate examples of cross-sectional structures of thethin film transistors. Each of a thin film transistors 425 and a thinfilm transistor 426 illustrated in FIGS. 7A and 7B is a thin filmtransistor with a structure in which an oxide semiconductor layer isinterposed between a conductive layer and a gate electrode layer.

In FIGS. 7A and 7B, a silicon substrate is used as a substrate, and thethin film transistor 425 and the thin film transistor 426 are eachprovided over an insulating layer 422 provided over a silicon substrate420.

In FIG. 7A, a conductive layer 427 is provided between the insulatinglayer 422 provided over the silicon substrate 420 and an insulatinglayer 407 so as to overlap with at least the entire oxide semiconductorlayer 412.

Note that FIG. 7B illustrates an example in which a conductive layerbetween the insulating layer 422 and the insulating layer 407 isprocessed into a conductive layer 424 by etching and overlaps with partof the oxide semiconductor layer 412, which includes at least a channelregion.

The conductive layer 427 and the conductive layer 424 may be formed of ametal material which can withstand the temperature of heat treatmentperformed later; as such a metal material, an element selected fromtitanium (Ti), tantalum (Ta), tungsten (W), molybdenum (Mo), chromium(Cr), neodymium (Nd), and scandium (Sc), an alloy containing any of theabove elements as a component, an alloy film containing a combination ofany of these elements, a nitride containing any of the above elements asits component, or the like can be used. In addition, the conductivelayers 427 and 424 may each have a single-layer structure or a stackedstructure. For example, a single-layer structure of a tungsten layer, astacked structure including a tungsten nitride layer and a tungstenlayer, or the like can be used.

Further, the potential of the conductive layers 427 and 424 may be thesame as or different from that of a gate electrode layer 411 of the thinfilm transistors 425 and 426. The conductive layers 427 and 424 can alsofunction as a second gate electrode layer. In addition, the potential ofthe conductive layers 427 and 424 may be fixed potential such as GND or0 V or may be in an electrically floating state without being connectedto anything.

The conductive layers 427 and 424 make it possible to control electriccharacteristics of the thin film transistors 425 and 426, respectively.

This embodiment can be implemented in combination with any of the otherembodiments as appropriate.

Here, a thin film transistor of an embodiment of the present inventionin which an oxide semiconductor is used is described with reference toenergy band diagrams.

FIG. 22 is a longitudinal section view of an inverted staggered thinfilm transistor in which an oxide semiconductor is used. An oxidesemiconductor layer (OS) is provided over a gate electrode (GE1) with agate insulating film (GI) interposed therebetween, and a sourceelectrode (S) and a drain electrode (D) are provided thereover.

FIGS. 23A and 23B are energy band diagrams (schematic diagrams) alongthe section A-A′ illustrated in FIG. 22. FIG. 23A illustrates the casewhere the voltage between the source and the drain is zero (VD=0 V, thepotential of the source and the potential of the drain are the same),and FIG. 23B illustrates the case where a positive potential withrespect to the source is applied to the drain (VD>0).

FIGS. 24A and 24B are energy band diagrams (schematic diagrams) alongthe section B-B′ illustrated in FIG. 22. FIG. 24A illustrates an onstate in which a positive potential (+VG) is applied to the gate (G1)and carriers (electrons) flow between the source and the drain. FIG. 24Billustrates an off state in which a negative potential (−VG) is appliedto the gate (G1) and minority carriers do not flow.

FIG. 25 shows the relations between the vacuum level and the workfunction of a metal (ϕM) and between the vacuum level and the electronaffinity (χ) of an oxide semiconductor.

Because metal is degenerated, the Fermi level is located in theconduction band. On the other hand, a conventional oxide semiconductoris typically an n-type semiconductor, in which case the Fermi level (Ef)is away from the intrinsic Fermi level (Ei) located in the middle of aband gap and is located closer to the conduction band. Note that it isknown that part of hydrogen is a donor in an oxide semiconductor and isone factor causing an oxide semiconductor to be an n-type semiconductor.

On the other hand, an oxide semiconductor of the present invention is anintrinsic (I-type) or a substantially intrinsic oxide semiconductorwhich is obtained by removing hydrogen that is an n-type impurity froman oxide semiconductor and purifying the oxide semiconductor so that animpurity other than main components of the oxide semiconductor isprevented from being contained therein as much as possible. In otherwords, a feature is that a purified I-type (intrinsic) semiconductor, ora semiconductor close thereto, is obtained not by adding an impurity butby removing an impurity such as hydrogen or water as much as possible.This enables the Fermi level (Ef) to be at the same level as theintrinsic Fermi level (Ei).

In the case where the band gap (Eg) of an oxide semiconductor is 3.15eV, the electron affinity (χ) is said to be 4.3 eV. The work function oftitanium (Ti) included in the source electrode and the drain electrodeis substantially equal to the electron affinity (χ) of the oxidesemiconductor. In that case, a Schottky barrier for electrons is notformed at an interface between the metal and the oxide semiconductor.

In other words, in the case where the work function of metal (ϕM) andthe electron affinity (χ) of the oxide semiconductor are equal to eachother and the metal and the oxide semiconductor are in contact with eachother, an energy band diagram (a schematic diagram) as illustrated inFIG. 23A is obtained.

In FIG. 23B, a black circle (●) represents an electron, and when apositive potential is applied to the drain, the electron is injectedinto the oxide semiconductor over the barrier (h) and flows toward thedrain. In that case, the height of the barrier (h) changes depending onthe gate voltage and the drain voltage; in the case where a positivedrain voltage is applied, the height of the barrier (h) is smaller thanthe height of the barrier in FIG. 23A where no voltage is applied, i.e.,½ of the band gap (Eg).

In this case, as shown in FIG. 24A, the electron moves along the lowestregion of the oxide semiconductor, which is energetically stable, at aninterface between the gate insulating film and the purified oxidesemiconductor.

In addition, in FIG. 24B, when a negative potential (reverse bias) isapplied to the gate electrode (G1), the value of current is extremelyclose to zero because holes that are minority carriers are substantiallyzero.

For example, even when a thin film transistor has a channel width W of1×10 μm and a channel length of 3 μm, the off-state current is 10⁻¹³ Aor less and the subthreshold swing (S value) is 0.1 V/dec (the thicknessof the gate insulating film: 100 nm).

As described above, the oxide semiconductor is purified so that theamount of impurities that are not main components of the oxidesemiconductor are minimized, whereby favorable operation of the thinfilm transistor can be obtained.

Embodiment 6

In this embodiment, an example of a thin film transistor which isincluded in the analog circuit described in Embodiment 1 will bedescribed.

An embodiment of the thin film transistor of this embodiment and amanufacturing method thereof will be described with reference to FIGS.8A to 8E.

A thin film transistor 310 illustrated in FIG. 8D is a thin filmtransistor having a bottom-gate structure and is also called an invertedstaggered thin film transistor.

Although the thin film transistor 310 is described as a single-gate thinfilm transistor, a multi-gate thin film transistor including a pluralityof channel regions can be formed when needed.

A process for manufacturing the thin film transistor 310 over asubstrate 300 will be described below with reference to FIGS. 8A to 8E.

First, a conductive film is formed over the substrate 300 having aninsulating surface, and a first photolithography step is performedthereon, so that a gate electrode layer 311 is formed. It is preferablethat an end portion of the formed gate electrode layer have a taperedshape to improve coverage with a gate insulating layer stackedthereover. Note that a resist mask may be formed by an ink-jet method.When the resist mask is formed by an ink-jet method, a photomask is notused; thus, the manufacturing cost can be reduced.

Although there is no particular limitation on a substrate which can beused as the substrate 300 having an insulating surface, the substrateneeds to have at least heat resistance high enough to withstand heattreatment to be performed later. As the substrate 300 having aninsulating surface, a glass substrate formed of barium borosilicateglass, aluminoborosilicate glass, or the like can be used.

In the case where a glass substrate is used and the temperature of theheat treatment to be performed later is high, a glass substrate whosestrain point is greater than or equal to 730° C. is preferably used. Asthe glass substrate, a substrate of a glass material such asaluminosilicate glass, aluminoborosilicate glass, or barium borosilicateglass is used, for example. Note that by containing a larger amount ofbarium oxide (BaO) than boron oxide (B₂O₃), a glass substrate that isheat-resistant and of more practical use can be obtained. Therefore, aglass substrate containing BaO and B₂O₃ so that the amount of BaO islarger than that of B₂O₃ is preferably used.

Note that a substrate formed of an insulator such as a ceramicsubstrate, a quartz substrate, or a sapphire substrate may be usedinstead of the above glass substrate. Alternatively, crystallized glassor the like can be used.

Further, an insulating film serving as a base film may be providedbetween the substrate 300 and the gate electrode layer 311. The basefilm has a function of preventing diffusion of an impurity element fromthe substrate 300, and can be formed to have a single-layer structure ora stacked structure using one or more films selected from a siliconnitride film, a silicon oxide film, a silicon nitride oxide film, and asilicon oxynitride film.

The gate electrode layer 311 can be formed to have a single-layerstructure or a stacked structure using a metal material such asmolybdenum, titanium, chromium, tantalum, tungsten, aluminum, copper,neodymium, or scandium or an alloy material which contains any of thesematerials as its main component.

For example, as a two-layer structure of the gate electrode layer 311,any of the following structures are preferable: a two-layer structure ofan aluminum layer and a molybdenum layer stacked thereover, a two-layerstructure of a copper layer and a molybdenum layer stacked thereover, atwo-layer structure of a copper layer and a titanium nitride layer or atantalum nitride layer stacked thereover, a two-layer structure of atitanium nitride layer and a molybdenum layer; and a two-layer structureof a tungsten nitride layer and a tungsten layer. As a three-layerstructure, a stack of a tungsten layer or a tungsten nitride layer, alayer of an alloy of aluminum and silicon or an alloy of aluminum andtitanium, and a titanium nitride layer or a titanium layer ispreferable. Note that the gate electrode layer can also be formed usinga light-transmitting conductive film. As a light-transmitting conductivefilm, a light-transmitting conductive oxide or the like can be given.

Next, a gate insulating layer 302 is formed over the gate electrodelayer 311.

The gate insulating layer 302 can be formed to have a single-layerstructure or a stacked structure of a silicon oxide layer, a siliconnitride layer, a silicon oxynitride layer, a silicon nitride oxidelayer, or an aluminum oxide layer by a plasma CVD method, a sputteringmethod, or the like. For example, a silicon oxynitride layer may beformed by a plasma CVD method using SiH₄, oxygen, and nitrogen as adeposition gas. The thickness of the gate insulating layer 302 isgreater than or equal to 100 nm and less than or equal to 500 nm; in thecase where the gate insulating layer 302 is formed to have a stackedstructure, for example, a first gate insulating layer with a thicknessgreater than or equal to 50 nm and less than or equal to 200 nm and asecond gate insulating layer with a thickness greater than or equal to 5nm and less than or equal to 300 nm are stacked.

In this embodiment, a silicon oxynitride layer having a thickness of 100nm or less is formed by a plasma CVD method as the gate insulating layer302.

Then, an oxide semiconductor film 330 is formed to a thickness ofgreater than or equal to 2 nm and less than or equal to 200 nm over thegate insulating layer 302.

As the oxide semiconductor film 330, any of the following is used: anIn—Ga—Zn—O-based oxide semiconductor film, an In—Sn—Zn—O-based oxidesemiconductor film, an In—Al—Zn—O-based oxide semiconductor film, aSn—Ga—Zn—O-based oxide semiconductor film, an Al—Ga—Zn—O-based oxidesemiconductor film, a Sn—Al—Zn—O-based oxide semiconductor film, anIn—Sn—O-based oxide semiconductor film, an In—Zn—O-based oxidesemiconductor film, a Sn—Zn—O-based oxide semiconductor film, anAl—Zn—O-based oxide semiconductor film, an In—Ga—O-based oxidesemiconductor film, an In—O-based oxide semiconductor film, a Sn—O-basedoxide semiconductor film, and a Zn—O-based oxide semiconductor film. Inthis embodiment, the oxide semiconductor film 330 is formed by asputtering method with use of an In—Ga—Zn—O-based metal oxide target. Across-sectional view at this stage is illustrated in FIG. 8A. The oxidesemiconductor film 330 can be formed by a sputtering method in a raregas (typically, argon) atmosphere, an oxygen atmosphere, or a mixedatmosphere including a rare gas (typically, argon) and oxygen. In thecase of using a sputtering method, a film may be formed using a targetincluding silicon oxide (SiO_(x) (x>0)) at 2 wt % to 10 wt % inclusive.

As a target for forming the oxide semiconductor film by a sputteringmethod, a metal oxide target containing zinc oxide as its main componentcan be used. As another example of the metal oxide target, a metal oxidetarget including In, Ga, and Zn (In₂O₃:Ga₂O₃:ZnO=1:1:1 in a molar ratio,In:Ga:Zn=1:1:0.5 in an atomic ratio) can be used. As the metal oxidetarget including In, Ga, and Zn, any of targets having the followingcomposition can also be used: In:Ga:Zn=1:1:1 or 1:1:2 in an atomicratio. The filling rate of the metal oxide target is higher than orequal to 90% and lower than or equal to 100%, and preferably higher thanor equal to 95% and lower than or equal to 99.9%. The use of the metaloxide target having a high filling rate makes it possible to form adense oxide semiconductor film.

As a sputtering gas used in formation of the oxide semiconductor film330, a high-purity gas in which the concentration of an impurity such ashydrogen, water, hydroxyl, or hydride is reduced to approximately theppm level or the ppb level is preferably used.

The substrate is held inside a treatment chamber which is kept in areduced pressure state, and the substrate temperature is set to behigher than or equal to 100° C. and lower than or equal to 600° C.,preferably higher than or equal to 200° C. and lower than or equal 15 to400° C. By heating the substrate during deposition, the impurityconcentration of the oxide semiconductor film formed can be reduced. Inaddition, damage by sputtering can be reduced. A sputtering gas fromwhich hydrogen and moisture have been removed is introduced whileresidual moisture in the treatment chamber is removed, and a metal oxideis used as a target. Thus, the oxide semiconductor film 330 is formedover the substrate 300. In order to remove residual moisture from thetreatment chamber, an adsorption-type vacuum pump is preferably used.For example, a cryopump, an ion pump, or a titanium sublimation pump ispreferably used. As an exhaustion unit, a turbo pump to which a coldtrap is added may be used. From the chamber in which exhaustion isperformed with the use of a cryopump, a hydrogen atom, a compoundincluding a hydrogen atom such as water (H₂O) (preferably, a compoundincluding a carbon atom as well), or the like, for example, isexhausted. Accordingly, the concentration of an impurity included in theoxide semiconductor film formed in the film formation chamber can bereduced.

As an example of film formation conditions, the following conditions areemployed: the distance between the substrate and the target is 100 mm,the pressure is 0.6 Pa, the direct-current (DC) power is 0.5 kW, and anoxygen atmosphere (the proportion of the oxygen flow is 100%) is used. Apulse direct current (DC) power supply is preferable because powdersubstances (also referred to as particles or dust) generated in the filmformation can be reduced and the film thickness can be made uniform. Theoxide semiconductor film preferably has a thickness greater than orequal to 2 nm and less than or equal to 200 nm and preferably has athickness greater than or equal to 5 nm and less than or equal to 30 nm.Note that appropriate thickness of the oxide semiconductor film variesdepending on the material; therefore, the thickness may be determined asappropriate depending on the material.

Next, the oxide semiconductor film 330 is processed into anisland-shaped oxide semiconductor layer in a second photolithographystep. A resist mask for forming the island-shaped oxide semiconductorlayer may be formed using an ink-jet method. When the resist mask isformed by an ink-jet method, a photomask is not used; thus, themanufacturing cost can be reduced.

Next in this embodiment, first heat treatment is performed on the oxidesemiconductor layer. Dehydration or dehydrogenation of the oxidesemiconductor layer can be performed through the first heat treatment.The temperature of the first heat treatment is higher than or equal to400° C. and lower than or equal to 750° C., preferably higher than orequal to 400° C. and lower than the strain point of the substrate. Here,the substrate is introduced into an electric furnace which is one ofheat treatment apparatuses, heat treatment is performed on the oxidesemiconductor layer in a nitrogen atmosphere at 450° C. for one hour,and then, the oxide semiconductor layer is not exposed to the air sothat entry of water and hydrogen into the oxide semiconductor layer isprevented; thus, an oxide semiconductor layer 331 is obtained (see FIG.8B).

The heat treatment apparatus is not limited to the electric furnace andmay be the one provided with a device for heating a process object usingheat conduction or heat radiation from a heating element such as aresistance heating element. For example, an RTA (rapid thermalannealing) apparatus such as a GRTA (gas rapid thermal annealing)apparatus or an LRTA (lamp rapid thermal annealing) apparatus can beused. An LRTA apparatus is an apparatus for heating a process object byradiation of light (an electromagnetic wave) emitted from a lamp such asa halogen lamp, a metal halide lamp, a xenon arc lamp, a carbon arclamp, a high-pressure sodium lamp, or a high-pressure mercury lamp. AGRTA apparatus is an apparatus for heat treatment using ahigh-temperature gas. As the gas, an inert gas which does not react witha process object by heat treatment, such as nitrogen or a rare gas suchas argon is used.

For example, as the first heat treatment, GRTA may be performed in thefollowing manner. The substrate is transferred and put in an inert gaswhich has been heated to a high temperature of 650° C. to 700° C.,heated for several minutes, and transferred and taken out of the inertgas which has been heated to a high temperature. GRTA enables ahigh-temperature heat treatment in a short time.

Note that in the first heat treatment, it is preferable that water,hydrogen, and the like be not contained in nitrogen or a rare gas suchas helium, neon, or argon. Alternatively, it is preferable that nitrogenor a rare gas such as helium, neon, or argon introduced into a heattreatment apparatus have a purity of 6N (99.9999%) or more, preferably,7N (99.99999%) or more (that is, the impurity concentration is set to 1ppm or lower, preferably, 0.1 ppm or lower).

Further, the oxide semiconductor layer may be crystallized to be amicrocrystalline layer or a polycrystalline layer depending on thecondition of the first heat treatment or a material of the oxidesemiconductor layer. For example, the oxide semiconductor layer may becrystallized to be a microcrystalline oxide semiconductor layer having adegree of crystallization of 90% or more, or 80% or more. The oxidesemiconductor layer may become an amorphous oxide semiconductor layercontaining no crystalline component depending on the condition of thefirst heat treatment or a material of the oxide semiconductor layer. Theoxide semiconductor layer may become an oxide semiconductor layer inwhich a microcrystalline portion (with a grain diameter greater than orequal to 1 nm and less than or equal to 20 nm, typically greater than orequal to 2 nm and less than or equal to 4 nm) is mixed into an amorphousoxide semiconductor.

The first heat treatment may be performed on the oxide semiconductorfilm 330 before being processed into the island-shaped oxidesemiconductor layer, instead of on the island-shaped oxide semiconductorlayer. In that case, after the first heat treatment, the substrate istaken out of the heating apparatus and a photolithography step isperformed.

The heat treatment having an effect of dehydrating or dehydrogenatingthe oxide semiconductor layer may be performed at any of the followingtimings: after the oxide semiconductor layer is formed; after a sourceelectrode and a drain electrode are formed over the oxide semiconductorlayer; and after a protective insulating film is formed over the sourceelectrode and the drain electrode.

Further, in the case where a contact hole is formed in the gateinsulating layer 302, the formation of the contact hole may be performedbefore or after the dehydration or dehydrogenation treatment of theoxide semiconductor film 330.

Note that the etching of the oxide semiconductor film may be dryetching, without limitation to wet etching.

The etching conditions (such as an etchant, etching time, andtemperature) are appropriately adjusted depending on the material sothat the material can be etched into a desired shape.

Next, a conductive film is formed over the gate insulating layer 302 andthe oxide semiconductor layer 331. The conductive film may be formed bya sputtering method or a vacuum evaporation method. As a material of theconductive film, an element selected from Al, Cr, Cu, Ta, Ti, Mo, and W;an alloy containing any of the above elements as its component; an alloyfilm containing these elements in combination; and the like can begiven. Alternatively, one or more materials selected from manganese,magnesium, zirconium, beryllium, and thorium may be used. The conductivefilm may have a single-layer structure or a stacked structure of two ormore layers. For example, a single-layer structure of an aluminum filmincluding silicon, a two-layer structure in which a titanium film isstacked over an aluminum film, a three-layer structure in which atitanium film, an aluminum film, and a titanium film are stacked in thisorder, and the like can be given. Alternatively, a film, an alloy film,or a nitride film which contains aluminum and one or a plurality ofelements selected from titanium, tantalum, tungsten, molybdenum,chromium, neodymium, and scandium may be used.

In the case where heat treatment is performed after formation of theconductive film, the conductive film preferably has heat resistanceenough to withstand the heat treatment.

A resist mask is formed over the conductive film by a thirdphotolithography step. Selective etching is performed, so that a sourceelectrode layer 315 a and a drain electrode layer 315 b are formed.Then, the resist mask is removed (see FIG. 8C).

For light exposure at the formation of the resist mask in the thirdphotolithography step, ultraviolet rays, KrF laser light, or ArF laserlight is used. The channel length L of the thin film transistor to becompleted later is determined by the distance between a lower endportion of the source electrode layer 315 a and a lower end portion ofthe drain electrode layer 315 b, which are adjacent to each other overthe oxide semiconductor layer 331. In the case where the channel lengthL is shorter than 25 nm and light exposure for forming the resist maskin the third photolithography step is performed, extreme ultravioletrays having a wavelength as extremely short as several nanometers toseveral tens of nanometers are used. Light exposure using extremeultraviolet rays has high resolution and large depth of focus.Therefore, the channel length L of the thin film transistor to becompleted later can be set at larger than or equal to 10 nm and lessthan or equal to 100 nm, whereby circuit operation speed is increased.In addition, since the off-state current is extremely low, lower powerconsumption can be achieved.

Note that each material and etching conditions are adjusted asappropriate so that the oxide semiconductor layer 331 is not removed byetching of the conductive film.

In this embodiment, a Ti film is used as the conductive film, anIn—Ga—Zn—O-based oxide semiconductor is used as the oxide semiconductorlayer 331, and an ammonia hydrogen peroxide solution (a mixture ofammonia, water, and a hydrogen peroxide solution) is used as an etchant.

Note that in the third photolithography step, the oxide semiconductorlayer 331 may be partly etched in some cases, so that an oxidesemiconductor layer having a groove (a depression portion) is formed. Aresist mask for forming the source electrode layer 315 a and the drainelectrode layer 315 b may be formed by an ink-jet method. When theresist mask is formed by an ink-jet method, a photomask is not used;thus, the manufacturing cost can be reduced.

Further, an oxide conductive layer may be formed between the oxidesemiconductor layer 331 and the source and drain electrode layers 315 aand 315 b. The oxide semiconductor layer 331 and a metal layer forforming the source electrode layer 315 a and the drain electrode layer315 b can be formed in succession. The oxide conductive layer canfunction as a source region and a drain region.

When the oxide conductive layer is provided as a source region and adrain region between the oxide semiconductor layer and the source anddrain electrode layers, the source region and the drain region can havelower resistance and the transistor can operate at high speed.

In order to reduce the number of photomasks and steps in aphotolithography step, etching may be performed with the use of a resistmask formed using a multi-tone mask which is a light-exposure maskthrough which light is transmitted so as to have a plurality ofintensities. Since a resist mask formed using a multi-tone mask has aplurality of thicknesses and can be further changed in shape byperforming etching, the resist mask can be used in a plurality ofetching steps to provide different patterns. Therefore, a resist maskcorresponding to at least two kinds of different patterns can be formedby using one multi-tone mask. Thus, the number of light-exposure maskscan be reduced and the number of corresponding photolithography stepscan also be reduced, whereby simplification of a process can berealized.

Then, plasma treatment using a gas such as N₂O, N₂, or Ar is performed.This plasma treatment removes water or the like adsorbed on a surface ofthe oxide semiconductor layer which is exposed. In addition, plasmatreatment may be performed using a mixed gas of oxygen and argon.

After the plasma treatment, the oxide insulating layer 316 serving as aprotective insulating film and is in contact with part of the oxidesemiconductor layer is formed without exposure to the air.

The oxide insulating layer 316 can be formed to a thickness of at least1 nm by a method by which an impurity such as water or hydrogen does notenter the oxide insulating layer 316, such as a sputtering method, asappropriate. When hydrogen is contained in the oxide insulating layer316, entry of the hydrogen to the oxide semiconductor layer orextraction of oxygen in the oxide semiconductor layer by the hydrogen iscaused, thereby making the resistance of the backchannel of the oxidesemiconductor layer low (to have an n-type conductivity), so that aparasitic channel might be formed. Therefore, it is important that aformation method in which hydrogen is not used is employed so that theoxide insulating layer 316 contains as little hydrogen as possible.

In this embodiment, a 200-nm-thick silicon oxide film is formed as theoxide insulating layer 316 by a sputtering method. The substratetemperature in film formation may be higher than or equal to roomtemperature and lower than or equal to 300° C. and in this embodiment,is 100° C. The formation of the silicon oxide film by a sputteringmethod can be performed in a rare gas (typically argon) atmosphere, anoxygen atmosphere, or an atmosphere of a rare gas (typically argon) andoxygen. As a target, a silicon oxide target or a silicon target may beused. For example, with the use of a silicon target, silicon oxide canbe formed by a sputtering method under an atmosphere of oxygen andnitrogen. As the oxide insulating layer 316 which is formed in contactwith the oxide semiconductor layer 331, an inorganic insulating filmwhich does not include impurities such as moisture, a hydrogen ion, orOH— and blocks entry of these from the outside is used. Typically, asilicon oxide film, a silicon oxynitride film, an aluminum oxide film,an aluminum oxynitride film, or the like is used.

In this case, the oxide insulating layer 316 is preferably formed whileresidual moisture in the treatment chamber is removed, in order toprevent hydrogen, hydroxyl, or moisture from being included in the oxidesemiconductor layer 331 and the oxide insulating layer 316.

In order to remove residual moisture from the treatment chamber, anadsorption-type vacuum pump is preferably used. For example, a cryopump,an ion pump, or a titanium sublimation pump is preferably used. As anexhaustion unit, a turbo pump to which a cold trap is added may be used.From the chamber in which exhaustion is performed with the use of acryopump, a hydrogen atom, a compound including a hydrogen atom such aswater (H₂O), or the like, for example, is exhausted. Accordingly, theconcentration of an impurity included in the oxide insulating layer 316formed in the film formation chamber can be reduced.

As a sputtering gas used in formation of the oxide insulating layer 316,a high-purity gas in which the concentration of an impurity such ashydrogen, water, hydroxyl, or hydride is reduced to approximately theppm level or the ppb level is preferably used.

Next, second heat treatment (preferably at 200° C. to 400° C., forexample at 250° C. to 350° C.) is performed in an inert gas atmosphereor an oxygen gas atmosphere. For example, the second heat treatment isperformed at 250° C. in a nitrogen atmosphere for one hour. In thesecond heat treatment, part of the oxide semiconductor layer (thechannel region) is heated in the state of being in contact with theoxide insulating layer 316. In the case where heat treatment isperformed in the state that the oxide semiconductor layer 331 and theoxide insulating layer 316 are in contact with each other, oxygen, whichis one of major components of the oxide semiconductor and is reduced atthe time of the first heat treatment, can be supplied from the oxideinsulating layer 316 to the oxide semiconductor layer 331. Thus, theoxide semiconductor is purified to become I-type (intrinsic).

Through the above-described process, the thin film transistor 310including the I-type oxide semiconductor layer 331 in which theconcentration of hydrogen, moisture, hydride, or hydroxide is reduced bydehydration or dehydrogenation can be formed (see FIG. 8D).

Furthermore, heat treatment may be performed at 100° C. to 200° C.inclusive for one hour to 30 hours in the air. In this embodiment, theheat treatment is performed at 150° C. for 10 hours. This heat treatmentmay be performed at a fixed heating temperature. Alternatively, thefollowing change in the heating temperature may be conducted pluraltimes repeatedly: the heating temperature is increased from roomtemperature to a temperature of 100° C. to 200° C. and then decreased toroom temperature. This heat treatment may be performed before theformation of the oxide insulating layer under a reduced pressure. Undera reduced pressure, the heat treatment time can be shortened. With thisheat treatment, a normally-off thin film transistor can be obtained.

A protective insulating layer 303 may be additionally formed over theoxide insulating layer 316. For example, a silicon nitride film isformed by an RF sputtering method. Since an RF sputtering method hashigh productivity, it is preferably used as a film formation method ofthe protective insulating layer. As the protective insulating layer, aninorganic insulating film which does not include impurities such asmoisture, a hydrogen ion, and OH— and blocks entry of these from theoutside is used. Typically, a silicon nitride film, an aluminum nitridefilm, a silicon nitride oxide film, an aluminum nitride oxide film, orthe like is used (see FIG. 8E).

In this embodiment, a silicon nitride film is formed as the protectiveinsulating layer 303 in such a manner that the substrate 300 over whichlayers up to the oxide insulating layer 316 are formed is heated to atemperature of 100° C. to 400° C., a sputtering gas which containshigh-purity nitrogen and from which hydrogen and moisture have beenremoved is used, and a silicon target is used. Also in this case, it ispreferable that the protective insulating layer 303 be formed whileresidual moisture in the treatment chamber is removed, in a mannersimilar to that of the oxide insulating layer 316.

A planarization insulating layer for planarization may be provided overthe protective insulating layer 303.

When the above-described thin film transistor is used in the analogcircuit described in Embodiment 1, the analog circuit can have stableelectric characteristics and high reliability.

This embodiment can be implemented in appropriate combination with anyof the other embodiments.

Embodiment 7

In this embodiment, an example of a thin film transistor which isincluded in the analog circuit described in Embodiment 1 will bedescribed.

An embodiment of the thin film transistor of this embodiment and amanufacturing method thereof will be described with reference to FIGS.9A to 9D.

A thin film transistor 360 illustrated in FIG. 9D is a kind ofbottom-gate thin film transistor called a channel-protective(channel-stop) thin film transistor and is also called an invertedstaggered thin film transistor.

Although the thin film transistor 360 is described as a single-gate thinfilm transistor, a multi-gate thin film transistor including a pluralityof channel regions can be formed when needed.

A process for manufacturing the thin film transistor 360 over asubstrate 320 will be described below with reference to FIGS. 9A to 9D.

First, a conductive film is formed over the substrate 320 having aninsulating surface, and a first photolithography step is performedthereon, so that a gate electrode layer 361 is formed. Note that aresist mask may be formed by an ink-jet method. When the resist mask isformed by an ink-jet method, a photomask is not used; thus, themanufacturing cost can be reduced.

The gate electrode layer 361 can be formed to have a single-layerstructure or a stacked structure using a metal material such asmolybdenum, titanium, chromium, tantalum, tungsten, aluminum, copper,neodymium, or scandium or an alloy material which contains any of thesematerials as its main component.

Next, a gate insulating layer 322 is formed over the gate electrodelayer 361.

In this embodiment, a silicon oxynitride layer having a thickness of 100nm or less is formed by a plasma CVD method as the gate insulating layer322.

Then, an oxide semiconductor film is formed to a thickness of greaterthan or equal to 2 nm and less than or equal to 200 nm over the gateinsulating layer 322, and the oxide semiconductor film is processed intoan island-shaped oxide semiconductor layer 332 by a secondphotolithography step. In this embodiment, the oxide semiconductor filmis formed by a sputtering method with use of an In—Ga—Zn—O-based metaloxide target.

In this case, the oxide semiconductor film is preferably formed whileresidual moisture in the treatment chamber is removed, in order toprevent hydrogen, hydroxyl, or moisture from being included in the oxidesemiconductor film.

In order to remove residual moisture from the treatment chamber, anadsorption-type vacuum pump is preferably used. For example, a cryopump,an ion pump, or a titanium sublimation pump is preferably used. As anexhaustion unit, a turbo pump to which a cold trap is added may be used.From the chamber in which exhaustion is performed with the use of acryopump, a hydrogen atom, a compound including a hydrogen atom such aswater (H₂O), or the like, for example, is exhausted. Accordingly, theconcentration of an impurity included in the oxide semiconductor filmformed in the film formation chamber can be reduced.

As a sputtering gas used in formation of the oxide semiconductor film, ahigh-purity gas in which the concentration of an impurity such ashydrogen, water, hydroxyl, or hydride is reduced to approximately theppm level or the ppb level is preferably used.

Next, dehydration or dehydrogenation of the oxide semiconductor layer332 is performed. The temperature of first heat treatment fordehydration or dehydrogenation is higher than or equal to 400° C. andlower than or equal to 750° C., preferably higher than or equal to 400°C. and lower than the strain point of the substrate. Here, the substrateis introduced into an electric furnace which is one of heat treatmentapparatuses, heat treatment is performed on the oxide semiconductorlayer 332 in a nitrogen atmosphere at 450° C. for one hour, and then,the oxide semiconductor layer 332 is not exposed to the air so thatentry of water and hydrogen into the oxide semiconductor layer 332 isprevented; thus, the dehydrated or dehydrogenated oxide semiconductorlayer 332 is obtained (see FIG. 9A).

Then, plasma treatment using a gas such as N₂O, N₂, or Ar is performed.This plasma treatment removes water or the like adsorbed on a surface ofthe oxide semiconductor layer which is exposed. In addition, plasmatreatment may be performed using a mixed gas of oxygen and argon.

Next, an oxide insulating layer is formed over the gate insulating layer322 and the oxide semiconductor layer 332, and then a resist mask isformed by a third photolithography step. Selective etching is performed,so that an oxide insulating layer 366 functioning as a channelprotective layer is formed. Then, the resist mask is removed. The oxideinsulating layer 366 serving as a channel protective layer can prevent aportion of the oxide semiconductor layer 332, which serves as a channelformation region later, from being damaged in a later step (for example,reduction in thickness due to plasma or an etchant in etching).

In this embodiment, a 200-nm-thick silicon oxide film is formed as theoxide insulating layer 366 by a sputtering method. The substratetemperature in film formation may be higher than or equal to roomtemperature and lower than or equal to 300° C. and in this embodiment,is 100° C. The formation of the silicon oxide film by a sputteringmethod can be performed in a rare gas (typically argon) atmosphere, anoxygen atmosphere, or an atmosphere of a rare gas (typically argon) andoxygen. As a target, a silicon oxide target or a silicon target may beused. For example, with the use of a silicon target, silicon oxide canbe formed by a sputtering method under an atmosphere of oxygen andnitrogen.

In this case, the oxide insulating layer 366 is preferably formed whileresidual moisture in the treatment chamber is removed, in order toprevent hydrogen, hydroxyl, or moisture from being included in the oxidesemiconductor layer 332 and the oxide insulating layer 366.

In order to remove residual moisture from the treatment chamber, anadsorption-type vacuum pump is preferably used. For example, a cryopump,an ion pump, or a titanium sublimation pump is preferably used. As anexhaustion unit, a turbo pump to which a cold trap is added may be used.From the chamber in which exhaustion is performed with the use of acryopump, a hydrogen atom, a compound including a hydrogen atom such aswater (H₂O), or the like, for example, is exhausted. Accordingly, theconcentration of an impurity included in the oxide insulating layer 366formed in the film formation chamber can be reduced.

As a sputtering gas used in formation of the oxide insulating layer 366,a high-purity gas in which the concentration of an impurity such ashydrogen, water, hydroxyl, or hydride is reduced to approximately theppm level or the ppb level is preferably used.

Next, second heat treatment (preferably at 200° C. to 400° C., forexample at 250° C. to 350° C.) may be performed in an inert gasatmosphere or an oxygen gas atmosphere. For example, the second heattreatment is performed at 250° C. in a nitrogen atmosphere for one hour.In the second heat treatment, part of the oxide semiconductor layer (thechannel region) is heated in the state of being in contact with theoxide insulating layer 366.

Next, a conductive film is formed over the gate insulating layer 322,the oxide semiconductor layer 332, and the oxide insulating layer 366,and then a resist mask is formed by a fourth photolithography step.Selective etching is performed, so that a source electrode layer 365 aand a drain electrode layer 365 b are formed. Then, the resist mask isremoved (see FIG. 9C).

As a material of the source electrode layer 365 a and the drainelectrode layer 365 b, an element selected from Al, Cr, Cu, Ta, Ti, Mo,and W; an alloy containing any of the above elements as its component;an alloy film containing these elements in combination; and the like canbe given. The metal conductive film may have a single-layer structure ora stacked structure of two or more layers. The second heat treatment maybe performed after the source electrode layer 365 a and the drainelectrode layer 365 b are formed.

Through the above-described process, the thin film transistor 360including the I-type oxide semiconductor layer 332 in which theconcentration of hydrogen, moisture, hydride, or hydroxide is reduced bydehydration or dehydrogenation is formed.

Furthermore, heat treatment may be performed at 100° C. to 200° C.inclusive for one hour to 30 hours in the air. In this embodiment, theheat treatment is performed at 150° C. for 10 hours. This heat treatmentmay be performed at a fixed heating temperature. Alternatively, thefollowing change in the heating temperature may be conducted pluraltimes repeatedly: the heating temperature is increased from roomtemperature to a temperature of 100° C. to 200° C. and then decreased toroom temperature. This heat treatment may be performed before theformation of the oxide insulating layer under a reduced pressure. Undera reduced pressure, the heat treatment time can be shortened. With thisheat treatment, a normally-off thin film transistor can be obtained.

Further, a protective insulating layer 323 may be formed over the sourceelectrode layer 365 a, the drain electrode layer 365 b, and the oxideinsulating layer 366. In this embodiment, the protective insulatinglayer 323 is formed using a silicon nitride film (see FIG. 9D).

Alternatively, an oxide insulating layer may be further formed over thesource electrode layer 365 a, the drain electrode layer 365 b, and theoxide insulating layer 366, and then the protective insulating layer 323may be stacked over the oxide insulating layer.

When the above-described thin film transistor is used in the analogcircuit described in Embodiment 1, the analog circuit can have stableelectric characteristics and high reliability.

This embodiment can be implemented in appropriate combination with anyof the other embodiments.

Embodiment 8

In this embodiment, an example of a thin film transistor which isincluded in the analog circuit described in Embodiment 1 will bedescribed.

An embodiment of the thin film transistor of this embodiment and amanufacturing method thereof will be described with reference to FIGS.10A to 10D.

Although a thin film transistor 350 illustrated in FIG. 10D will bedescribed as a single-gate thin film transistor, a multi-gate thin filmtransistor including a plurality of channel regions can be formed whenneeded.

A process for manufacturing the thin film transistor 350 over asubstrate 340 will be described below with reference to FIGS. 10A to10D.

First, a conductive film is formed over the substrate 340 having aninsulating surface, and a first photolithography step is performedthereon, so that a gate electrode layer 351 is formed. In thisembodiment, a tungsten film having a thickness of 150 nm is formed by asputtering method as the gate electrode layer 351.

Next, a gate insulating layer 342 is formed over the gate electrodelayer 351. In this embodiment, a silicon oxynitride layer having athickness of 100 nm or less is formed by a plasma CVD method as the gateinsulating layer 342.

Next, a conductive film is formed over the gate insulating layer 342. Aresist mask is formed over the conductive film by a secondphotolithography step. Selective etching is performed, so that a sourceelectrode layer 355 a and a drain electrode layer 355 b are formed.Then, the resist mask is removed (see FIG. 10A).

Next, an oxide semiconductor film 345 is formed (see FIG. 10B). In thisembodiment, the oxide semiconductor film 345 is formed by a sputteringmethod with use of an In—Ga—Zn—O-based metal oxide target. The oxidesemiconductor film 345 is processed into an island-shaped oxidesemiconductor layer 346 by a third photolithography step.

In this case, the oxide semiconductor film 345 is preferably formedwhile residual moisture in the treatment chamber is removed, in order toprevent hydrogen, hydroxyl, or moisture from being included in the oxidesemiconductor film 345.

In order to remove residual moisture from the treatment chamber, anadsorption-type vacuum pump is preferably used. For example, a cryopump,an ion pump, or a titanium sublimation pump is preferably used. As anexhaustion unit, a turbo pump to which a cold trap is added may be used.From the chamber in which exhaustion is performed with the use of acryopump, a hydrogen atom, a compound including a hydrogen atom such aswater (H₂O), or the like, for example, is exhausted. Accordingly, theconcentration of an impurity included in the oxide semiconductor film345 formed in the film formation chamber can be reduced.

As a sputtering gas used in formation of the oxide semiconductor film345, a high-purity gas in which the concentration of an impurity such ashydrogen, water, hydroxyl, or hydride is reduced to approximately theppm level or the ppb level is preferably used.

Next, dehydration or dehydrogenation of the oxide semiconductor layer346 is performed. The temperature of first heat treatment fordehydration or dehydrogenation is higher than or equal to 400° C. andlower than or equal to 750° C., preferably higher than or equal to 400°C. and lower than the strain point of the substrate. Here, the substrateis introduced into an electric furnace which is one of heat treatmentapparatuses, heat treatment is performed on the oxide semiconductorlayer 346 in a nitrogen atmosphere at 450° C. for one hour, and then,the oxide semiconductor layer 346 is not exposed to the air so thatentry of water and hydrogen into the oxide semiconductor layer 346 isprevented; thus, the dehydrated or dehydrogenated oxide semiconductorlayer 346 is obtained (see FIG. 10C).

As the first heat treatment, GRTA may be performed in the followingmanner. The substrate is transferred and put in an inert gas which hasbeen heated to a high temperature of 650° C. to 700° C., heated forseveral minutes, and transferred and taken out of the inert gas whichhas been heated to a high temperature. GRTA enables a high-temperatureheat treatment in a short time.

An oxide insulating layer 356 serving as a protective insulating film isformed in contact with the oxide semiconductor layer 346.

The oxide insulating layer 356 can be formed to a thickness of at least1 nm by a method by which an impurity such as water or hydrogen does notenter the oxide insulating layer 356, such as a sputtering method, asappropriate. When hydrogen is contained in the oxide insulating layer356, entry of the hydrogen to the oxide semiconductor layer orextraction of oxygen in the oxide semiconductor layer by the hydrogen iscaused, thereby making the resistance of the backchannel of the oxidesemiconductor layer low (to have an n-type conductivity), so that aparasitic channel might be formed. Therefore, it is important that aformation method in which hydrogen is not used is employed so that theoxide insulating layer 356 contains as little hydrogen as possible.

In this embodiment, a 200-nm-thick silicon oxide film is formed as theoxide insulating layer 356 by a sputtering method. The substratetemperature in film formation may be higher than or equal to roomtemperature and lower than or equal to 300° C. and in this embodiment,is 100° C. The formation of the silicon oxide film by a sputteringmethod can be performed in a rare gas (typically argon) atmosphere, anoxygen atmosphere, or an atmosphere of a rare gas (typically argon) andoxygen. As a target, a silicon oxide target or a silicon target may beused. For example, with the use of a silicon target, silicon oxide canbe formed by a sputtering method under an atmosphere of oxygen andnitrogen. As the oxide insulating layer 356 which is formed in contactwith the oxide semiconductor layer 346, an inorganic insulating filmwhich does not include impurities such as moisture, a hydrogen ion, orOH— and blocks entry of these from the outside is used. Typically, asilicon oxide film, a silicon oxynitride film, an aluminum oxide film,an aluminum oxynitride film, or the like is used.

In this case, the oxide insulating layer 356 is preferably formed whileresidual moisture in the treatment chamber is removed, in order toprevent hydrogen, hydroxyl, or moisture from being included in the oxidesemiconductor layer 346 and the oxide insulating layer 356.

In order to remove residual moisture from the treatment chamber, anadsorption-type vacuum pump is preferably used. For example, a cryopump,an ion pump, or a titanium sublimation pump is preferably used. As anexhaustion unit, a turbo pump to which a cold trap is added may be used.From the chamber in which exhaustion is performed with the use of acryopump, a hydrogen atom, a compound including a hydrogen atom such aswater (H₂O), or the like, for example, is exhausted. Accordingly, theconcentration of an impurity included in the oxide insulating layer 356formed in the film formation chamber can be reduced.

As a sputtering gas used in formation of the oxide insulating layer 356,a high-purity gas in which the concentration of an impurity such ashydrogen, water, hydroxyl, or hydride is reduced to approximately theppm level or the ppb level is preferably used.

Next, second heat treatment (preferably at 200° C. to 400° C., forexample at 250° C. to 350° C.) is performed in an inert gas atmosphereor an oxygen gas atmosphere. For example, the second heat treatment isperformed at 250° C. in a nitrogen atmosphere for one hour. The secondheat treatment is performed in the state that the oxide semiconductorlayer is in contact with the oxide insulating layer 356.

Through the above-described process, the thin film transistor 350including the I-type oxide semiconductor layer 346 in which theconcentration of hydrogen, moisture, hydride, or hydroxide is reduced bydehydration or dehydrogenation can be formed.

Furthermore, heat treatment may be performed at 100° C. to 200° C.inclusive for one hour to 30 hours in the air. In this embodiment, theheat treatment is performed at 150° C. for 10 hours. This heat treatmentmay be performed at a fixed heating temperature. Alternatively, thefollowing change in the heating temperature may be conducted pluraltimes repeatedly: the heating temperature is increased from roomtemperature to a temperature of 100° C. to 200° C. and then decreased toroom temperature. This heat treatment may be performed before theformation of the oxide insulating layer under a reduced pressure. Undera reduced pressure, the heat treatment time can be shortened. With thisheat treatment, a normally-off thin film transistor can be obtained.

A protective insulating layer may be additionally formed over the oxideinsulating layer 356. For example, a silicon nitride film is formed byan RF sputtering method. In this embodiment, a protective insulatinglayer 343 serving as a protective insulating layer is formed using asilicon nitride film (see FIG. 10D).

A planarization insulating layer for planarization may be provided overthe protective insulating layer 343.

When the above-described thin film transistor is used in the analogcircuit described in Embodiment 1, the analog circuit can have stableelectric characteristics and high reliability.

This embodiment can be implemented in appropriate combination with anyof the other embodiments.

Embodiment 9

In this embodiment, an example of a thin film transistor which isincluded in the analog circuit described in Embodiment 1 will bedescribed.

In this embodiment, an example which is different from Embodiment 6 inpart of the manufacturing process of a thin film transistor will bedescribed with reference to FIG. 11. Since FIG. 11 is the same as FIGS.8A to 8E except for part of the process, the same reference numerals areused for the same portions, and detailed description of the sameportions is not repeated.

In accordance with Embodiment 6, a gate electrode layer 381 is formedover a substrate 370, and a first gate insulating layer 372 a and asecond gate insulating layer 372 b are stacked. In this embodiment, thegate insulating layer has a two-layer structure: a nitride insulatinglayer is used for the first gate insulating layer 372 a and an oxideinsulating layer is used for the second gate insulating layer 372 b.

As the oxide insulating layer, a silicon oxide layer, a siliconoxynitride layer, an aluminum oxide layer, an aluminum oxynitride layer,or the like can be used. Further, as the nitride insulating layer, asilicon nitride layer, a silicon nitride oxide layer, an aluminumnitride layer, an aluminum nitride oxide layer, or the like can be used.

In this embodiment, the gate insulating layer has a structure in which asilicon nitride layer and a silicon oxide layer are stacked in thisorder over the gate electrode layer 381. A gate insulating layer havinga thickness of 150 nm is formed in such a manner that a silicon nitridelayer (SiN_(y) (y>0)) having a thickness of 50 nm to 200 nm inclusive(50 nm in this embodiment) is formed as the first gate insulating layer372 a by a sputtering method and a silicon oxide layer (SiO_(x) (x>0))having a thickness of 5 nm to 300 nm inclusive (100 nm in thisembodiment) is stacked as the second gate insulating layer 372 b overthe first gate insulating layer 372 a.

Next, an oxide semiconductor film is formed and is processed into anisland-shaped oxide semiconductor layer 382 by a photolithography step.In this embodiment, the oxide semiconductor film is formed by asputtering method with use of an In—Ga—Zn—O-based metal oxide target.

In this case, the oxide insulating film is preferably formed whileresidual moisture in the treatment chamber is removed, in order toprevent hydrogen, hydroxyl, or moisture from being included in the oxidesemiconductor film.

In order to remove residual moisture from the treatment chamber, anadsorption-type vacuum pump is preferably used. For example, a cryopump,an ion pump, or a titanium sublimation pump is preferably used. As anexhaustion unit, a turbo pump to which a cold trap is added may be used.From the chamber in which exhaustion is performed with the use of acryopump, a hydrogen atom, a compound including a hydrogen atom such aswater (H₂O), or the like, for example, is exhausted. Accordingly, theconcentration of an impurity included in the oxide semiconductor filmformed in the film formation chamber can be reduced.

As a sputtering gas used in formation of the oxide semiconductor film, ahigh-purity gas in which the concentration of an impurity such ashydrogen, water, hydroxyl, or hydride is reduced to approximately theppm level or the ppb level is preferably used.

Next, dehydration or dehydrogenation of the oxide semiconductor layer382 is performed. The temperature of first heat treatment fordehydration or dehydrogenation is higher than or equal to 400° C. andlower than or equal to 750° C., preferably higher than or equal to 425°C. When the temperature is 425° C. or higher, the heat treatment timemay be one hour or less, whereas when the temperature is lower than 425°C., the heat treatment time is longer than one hour. Here, the substrateis introduced into an electric furnace which is one of heat treatmentapparatuses, heat treatment is performed on the oxide semiconductorlayer in a nitrogen atmosphere, and then, the oxide semiconductor layeris not exposed to the air so that entry of water and hydrogen into theoxide semiconductor layer is prevented; thus, the oxide semiconductorlayer is obtained. After that, cooling is performed by introduction of ahigh-purity oxygen gas, a high-purity N₂O gas, or ultra-dry air (havinga dew point of −40° C. or lower, preferably −60° C. or lower) into thesame furnace. It is preferable that the oxygen gas and the N₂O gas donot include water, hydrogen, and the like. The purity of an oxygen gasor an N₂O gas which is introduced into the heat treatment apparatus ispreferably 6N (99.9999%) or higher, further preferably 7N (99.99999%) orhigher (that is, the impurity concentration of the oxygen gas or the N₂Ogas is 1 ppm or lower, preferably 0.1 ppm or lower).

The heat treatment apparatus is not limited to the electric furnace and,for example, an RTA (rapid thermal annealing) apparatus such as a GRTA(gas rapid thermal annealing) apparatus or an LRTA (lamp rapid thermalannealing) apparatus can be used. An LRTA apparatus is an apparatus forheating a process object by radiation of light (an electromagnetic wave)emitted from a lamp such as a halogen lamp, a metal halide lamp, a xenonarc lamp, a carbon arc lamp, a high-pressure sodium lamp, or ahigh-pressure mercury lamp. In addition, the LRTA apparatus may beprovided with not only a lamp but also a device which heats a processobject by heat conduction or heat radiation from a heater such as aresistance heater. GRTA is a method of heat treatment using ahigh-temperature gas. As the gas, an inert gas which does not react witha process object by heat treatment, such as nitrogen or a rare gas suchas argon is used. The heat treatment may be performed at 600° C. to 750°C. for several minutes using an RTA method.

Further, after the first heat treatment for dehydration ordehydrogenation, heat treatment may be performed at 200° C. to 400° C.inclusive, preferably 200° C. to 300° C. inclusive, in an atmosphere ofan oxygen gas or an N₂O gas.

The first heat treatment may be performed on the oxide semiconductorfilm before being processed into the island-shaped oxide semiconductorlayer, instead of on the island-shaped oxide semiconductor layer. Inthat case, after the first heat treatment, the substrate is taken out ofthe heating apparatus and a photolithography step is performed.

Through the above-described process, impurities in the oxidesemiconductor are reduced, whereby the oxide semiconductor layer 382whose entire region is I-type or substantially I-type can be obtained.

Next, a conductive film is formed over the oxide semiconductor layer382, a resist mask is formed by a photolithography step, and theconductive film is selectively etched, so that a source electrode layer385 a and a drain electrode layer 385 b are formed. Then, an oxideinsulating layer 386 is formed by a sputtering method.

In this case, the oxide insulating layer 386 is preferably formed whileresidual moisture in the treatment chamber is removed, in order toprevent hydrogen, hydroxyl, or moisture from being included in the oxidesemiconductor layer 382 and the oxide insulating layer 386.

In order to remove residual moisture from the treatment chamber, anadsorption-type vacuum pump is preferably used. For example, a cryopump,an ion pump, or a titanium sublimation pump is preferably used. As anexhaustion unit, a turbo pump to which a cold trap is added may be used.From the chamber in which exhaustion is performed with the use of acryopump, a hydrogen atom, a compound including a hydrogen atom such aswater (H₂O), or the like, for example, is exhausted. Accordingly, theconcentration of an impurity included in the oxide insulating layer 386formed in the film formation chamber can be reduced.

As a sputtering gas used in formation of the oxide insulating layer 386,a high-purity gas in which the concentration of an impurity such ashydrogen, water, hydroxyl, or hydride is reduced to approximately theppm level or the ppb level is preferably used.

Through the above-described process, a thin film transistor 380 can beformed.

Next, in order to reduce variation in electric characteristics of thethin film transistor, heat treatment (preferably at higher than or equalto 150° C. and lower than 350° C.) may be performed in an inert gasatmosphere or a nitrogen gas atmosphere. For example, the heat treatmentis performed at 250° C. in a nitrogen atmosphere for one hour.

Further, heat treatment may be performed at 100° C. to 200° C. inclusivefor one hour to 30 hours in the air. In this embodiment, the heattreatment is performed at 150° C. for 10 hours. This heat treatment maybe performed at a fixed heating temperature. Alternatively, thefollowing change in the heating temperature may be conducted pluraltimes repeatedly: the heating temperature is increased from roomtemperature to a temperature of 100° C. to 200° C. and then decreased toroom temperature. This heat treatment may be performed before theformation of the oxide insulating layer under a reduced pressure. Undera reduced pressure, the heat treatment time can be shortened. With thisheat treatment, a normally-off thin film transistor can be obtained.

A protective insulating layer 373 is formed over the oxide insulatinglayer 386. In this embodiment, a silicon nitride film having a thicknessof 100 nm is formed by a sputtering method as the protective insulatinglayer 373.

The protective insulating layer 373 and the first gate insulating layer372 a, which are formed using a nitride insulating layer, do not includeimpurities such as moisture, hydrogen, hydride, or hydroxide and have aneffect of blocking entry of these from the outside.

Accordingly, entry of impurities such as moisture from the outside canbe prevented in a manufacturing process after formation of theprotective insulating layer 373. Further, even after a device iscompleted as a semiconductor device such as a liquid crystal displaydevice, entry of impurities such as moisture from the outside can beprevented in the long term; therefore, long-term reliability of thedevice can be improved.

Further, a structure in which the protective insulating layer 373 is incontact with the first gate insulating layer 372 a by removing aninsulating layer provided between the protective insulating layer 373and the first gate insulating layer 372 a, which are formed using anitride insulating layer.

With the structure in which the protective insulating layer 373 is incontact with the first gate insulating layer 372 a, impurities such asmoisture, hydrogen, hydride, or hydroxide in the oxide semiconductorlayer can be reduced to a minimum amount, and in addition, theimpurities can be prevented from entering the oxide semiconductor layeragain. Accordingly, the impurity concentration in the oxidesemiconductor layer can be kept low.

A planarization insulating layer for planarization may be provided overthe protective insulating layer 373.

When the above-described thin film transistor is used in the analogcircuit described in Embodiment 1, the analog circuit can have stableelectric characteristics and high reliability.

This embodiment can be implemented in appropriate combination with anyof the other embodiments.

Embodiment 10

In this embodiment, an example of a semiconductor device including theanalog circuit described in Embodiment 1 will be described.Specifically, the appearance and a cross section of a liquid crystaldisplay panel including the photodetector described in Embodiment 1 willbe described with reference to FIGS. 12A to 12C. FIGS. 12A and 12C areplan views of panels in which thin film transistors 4010 and 4011 and aliquid crystal element 4013 are sealed between a first substrate 4001and a second substrate 4006 with a sealant 4005. FIG. 12B is across-sectional view taken along M-N in FIG. 12A or FIG. 12C.

The sealant 4005 is provided so as to surround a pixel portion 4002 anda scan line driver circuit 4004 which are provided over the firstsubstrate 4001. The second substrate 4006 is provided over the pixelportion 4002 and the scan line driver circuit 4004. Consequently, thepixel portion 4002 and the scan line driver circuit 4004 are sealedtogether with a liquid crystal layer 4008, by the first substrate 4001,the sealant 4005, and the second substrate 4006. A signal line drivercircuit 4003 that is formed using a single crystal semiconductor film ora polycrystalline semiconductor film over a substrate separatelyprepared is mounted in a region that is different from the regionsurrounded by the sealant 4005 over the first substrate 4001.

A photodetector 4100 described in Embodiment 1 is provided in a regionthat is different from the region surrounded by the sealant 4005 overthe first substrate 4001. The photodetector 4100 may be formed over thefirst substrate 4001 at the same time as the pixel portion or may beformed over a different substrate and then mounted on the firstsubstrate 4001. Note that in the case of using a light-transmittingsubstrate as the first substrate 4001, the photodetector 4100 can have astructure of detecting light that enters from the substrate side, whilein the case of using a substrate that does not transmit visible light asthe first substrate 4001, a light-receiving portion of the photodetectorneeds to face in such a direction as not to be influenced by blocking oflight due to the substrate.

Note that there is no particular limitation on the connection method ofthe driver circuit which is separately formed, and a COG method, a wirebonding method, a TAB method, or the like can be used. FIG. 12Aillustrates an example in which the signal line driver circuit 4003 ismounted by a COG method. FIG. 12C illustrates an example in which thesignal line driver circuit 4003 is mounted by a TAB method.

The pixel portion 4002 and the scan line driver circuit 4004 providedover the first substrate 4001 include a plurality of thin filmtransistors. FIG. 12B illustrates the thin film transistor 4010 includedin the pixel portion 4002 and the thin film transistor 4011 included inthe scan line driver circuit 4004, as an example. An insulating layer4041, a protective insulating layer 4042, an insulating layer 4020, andan insulating layer 4021 are provided over the thin film transistors4010 and 4011.

Any of the thin film transistors of Embodiments 3 to 9 can be used asappropriate as the thin film transistors 4010 and 4011, and they can beformed using steps and materials similar to those for the thin filmtransistors of Embodiments 3 to 9. The thin film transistors 4010 and4011 each include an oxide semiconductor layer in which hydrogen orwater is reduced. Therefore, the thin film transistors 4010 and 4011 arehighly reliable thin film transistors. In this embodiment, the thin filmtransistors 4010 and 4011 are n-channel thin film transistors.

A conductive layer 4040 is provided over part of the insulating layer4021, which overlaps with a channel region of an oxide semiconductorlayer in the thin film transistor 4011 for the driver circuit. Theconductive layer 4040 is provided in the position overlapping with thechannel region of the oxide semiconductor layer, whereby the amount ofchange in threshold voltage of the thin film transistor 4011 before andafter the BT test can be reduced. A potential of the conductive layer4040 may be the same or different from that of a gate electrode layer ofthe thin film transistor 4011. The conductive layer 4040 can alsofunction as a second gate electrode layer. Further, the potential of theconductive layer 4040 may be GND or 0 V, or the conductive layer 4040may be in a floating state. Note that it is acceptable if the conductivelayer 4040 is not provided.

A pixel electrode layer 4030 included in the liquid crystal element 4013is electrically connected to a source or drain electrode layer of thethin film transistor 4010. A counter electrode layer 4031 of the liquidcrystal element 4013 is formed on the second substrate 4006. A portionwhere the pixel electrode layer 4030, the counter electrode layer 4031,and the liquid crystal layer 4008 overlap with one another correspondsto the liquid crystal element 4013. Note that the pixel electrode layer4030 and the counter electrode layer 4031 are provided with aninsulating layer 4032 and an insulating layer 4033 functioning asalignment films, respectively, and the liquid crystal layer 4008 issandwiched between the electrode layers with the insulating layers 4032and 4033 therebetween.

Note that a light-transmitting substrate can be used as the firstsubstrate 4001 and the second substrate 4006; glass, ceramics, orplastics can be used. The plastic may be a fiberglass-reinforcedplastics (FRP) plate, a polyvinyl fluoride (PVF) film, a polyester film,or an acrylic resin film.

A spacer 4035 is a columnar spacer obtained by selective etching of aninsulating film, and the columnar spacer 4035 is provided in order tocontrol the distance (a cell gap) between the pixel electrode layer 4030and the counter electrode layer 4031. Alternatively, a spherical spacermay be used as the spacer 4035. The counter electrode layer 4031 iselectrically connected to a common potential line formed over thesubstrate where the thin film transistor 4010 is formed. The counterelectrode layer 4031 and the common potential line can be electricallyconnected to each other through conductive particles provided betweenthe pair of substrates using the common connection portion. Note thatthe conductive particles are included in the sealant 4005.

Alternatively, a liquid crystal exhibiting a blue phase for which analignment film is unnecessary may be used. A blue phase is one of liquidcrystal phases, and is generated just before a cholesteric phase changesinto an isotropic phase while the temperature of cholesteric liquidcrystal is increased. Since the blue phase is only generated within anarrow range of temperature, a liquid crystal composition containing achiral agent at 5 wt % or more is used for the liquid crystal layer 4008in order to improve the temperature range. The liquid crystalcomposition including liquid crystal exhibiting a blue phase and achiral agent has a short response time of 1 msec or less and isoptically isotropic; therefore, alignment treatment is not necessary andviewing angle dependence is small. In addition, since an alignment filmdoes not need to be provided and rubbing treatment is unnecessary,electrostatic discharge damage caused by the rubbing treatment can beprevented and defects and damage of the liquid crystal display devicecan be reduced in the manufacturing process. Thus, productivity of theliquid crystal display device can be increased. A thin film transistorthat uses an oxide semiconductor layer particularly has a possibilitythat electric characteristics of the thin film transistor may fluctuatesignificantly by the influence of static electricity and deviate fromthe designed range. Therefore, it is more effective to use a blue phaseliquid crystal material for a liquid crystal display device including athin film transistor that uses an oxide semiconductor layer.

Note that an embodiment of the present invention can also be applied toa transflective liquid crystal display device in addition to atransmissive liquid crystal display device.

Although a polarizing plate is provided on the outer surface of thesubstrate (on the viewer side) and a coloring layer and an electrodelayer used for a display element are sequentially provided on the innersurface of the substrate in the example of the liquid crystal displaydevice, the polarizing plate may be provided on the inner surface of thesubstrate. The stacked structure of the polarizing plate and thecoloring layer is not limited to that in this embodiment and may be setas appropriate depending on materials of the polarizing plate and thecoloring layer or conditions of the manufacturing process. Further, alight-blocking film serving as a black matrix may be provided in aportion other than the display portion.

Over the thin film transistors 4011 and 4010, the insulating layer 4041is formed in contact with the oxide semiconductor layers. The insulatinglayer 4041 can be formed using a material and a method similar to thoseof the oxide insulating layer described in any of the other embodiments.Here, as the insulating layer 4041, a silicon oxide layer is formed by asputtering method. Further, the protective insulating layer 4042 isformed on and in contact with the insulating layer 4041. The protectiveinsulating layer 4042 can be formed in a manner similar to that of theprotective insulating layer described in any of the other embodiments,and a silicon nitride film can be used, for example. In order to reducethe surface roughness caused by the thin film transistors, theinsulating layer 4021 serving as a planarization film is formed over theprotective insulating layer 4042.

As the insulating layer 4021 functioning as a planarization insulatingfilm, an organic material having heat resistance such as polyimide, anacrylic resin, a benzocyclobutene-based resin, polyamide, or an epoxyresin can be used. Other than such organic materials, it is possible touse a low-dielectric constant material (a low-k material), asiloxane-based resin, PSG (phosphosilicate glass), BPSG(borophosphosilicate glass), or the like. Note that the insulating layer4021 may be formed by stacking a plurality of insulating films formed ofthese materials.

There is no particular limitation on the method for forming theinsulating layer 4021. The insulating layer 4021 can be formed,depending on the material, by a method such as a sputtering method, anSOG method, a spin coating method, a dipping method, a spray coatingmethod, or a droplet discharge method (e.g., an ink-jet method, screenprinting, or offset printing), or a tool (equipment) such as a doctorknife, a roll coater, a curtain coater, or a knife coater. A baking stepof the insulating layer 4021 also serves as annealing of thesemiconductor layer, whereby a semiconductor device can be manufacturedefficiently.

The pixel electrode layer 4030 and the counter electrode layer 4031 canbe formed using a light-transmitting conductive material such as indiumtin oxide (ITO), indium zinc oxide (IZO), a conductive material in whichsilicon oxide (SiO_(x) (x>0)) is mixed in indium oxide, organoindium,organotin, indium oxide containing tungsten oxide, indium zinc oxidecontaining tungsten oxide, indium oxide containing titanium oxide,indium tin oxide containing titanium oxide, or the like. Further, in thecase where a light-transmitting property is not needed or a reflectingproperty is needed in a reflective liquid crystal display device, thepixel electrode layer 4030 and the counter electrode layer 4031 can beformed using one or more kinds of materials selected from a metal suchas tungsten (W), molybdenum (Mo), zirconium (Zr), hafnium (Hf), vanadium(V), niobium (Nb), tantalum (Ta), chromium (Cr), cobalt (Co), nickel(Ni), titanium (Ti), platinum (Pt), aluminum (Al), copper (Cu), andsilver (Ag); an alloy of these metals; and a nitride of these metals.

A conductive composition containing a conductive molecule of highmolecular weight (also referred to as a conductive polymer) can be usedfor the pixel electrode layer 4030 and the counter electrode layer 4031.The pixel electrode formed using the conductive composition preferablyhas a sheet resistance of less than or equal to 10000 ohms per squareand a transmittance of greater than or equal to 70% at a wavelength of550 nm. Further, the resistivity of the conductive polymer contained inthe conductive composition is preferably less than or equal to 0.1 Ω·cm.

As the conductive polymer, a so-called x-electron conjugated conductivepolymer can be used. For example, polyaniline or a derivative thereof,polypyrrole or a derivative thereof, polythiophene or a derivativethereof, a copolymer of two or more kinds of them, and the like can begiven.

Further, a variety of signals and potentials are supplied to the signalline driver circuit 4003 which is formed separately, the scan linedriver circuit 4004, or the pixel portion 4002 from an FPC 4018.

A connection terminal electrode 4015 is formed from the same conductivefilm as the pixel electrode layer 4030 included in the liquid crystalelement 4013, and a terminal electrode 4016 is formed from the sameconductive film as source and drain electrode layers of the thin filmtransistors 4010 and 4011.

The connection terminal electrode 4015 is electrically connected to aterminal included in the FPC 4018 through an anisotropic conductive film4019.

Note that FIGS. 12A to 12C illustrate an example in which the signalline driver circuit 4003 is formed separately and mounted on the firstsubstrate 4001; however, the present invention is not limited to thisstructure. The scan line driver circuit may be separately formed andthen mounted, or only part of the signal line driver circuit or part ofthe scan line driver circuit may be separately formed and then mounted.

A black matrix (a light-blocking layer), an optical element (an opticalsubstrate) such as a polarizing member, a retardation member, or ananti-reflection member, and the like are provided as appropriate. Forexample, circular polarization may be employed by using a polarizingsubstrate and a retardation substrate. In addition, a backlight, asidelight, or the like may be used as a light source.

In an active matrix liquid crystal display device, display patterns areformed on a screen by driving of pixel electrodes that are arranged inmatrix. Specifically, voltage is applied between a selected pixelelectrode and a counter electrode corresponding to the pixel electrode,and thus, a liquid crystal layer disposed between the pixel electrodeand the counter electrode is optically modulated. This opticalmodulation is recognized as a display pattern by a viewer.

A liquid crystal display device has a problem in that, when displaying amoving image, image sticking occurs or the moving image is blurredbecause the response speed of liquid crystal molecules themselves islow. As a technique for improving moving image characteristics of aliquid crystal display device, there is a driving technique so-calledblack insertion by which an entirely black image is displayed everyother frame.

Alternatively, a driving method called double-frame rate driving may beemployed in which a vertical synchronizing frequency is 1.5 times ormore, preferably 2 times or more as high as a normal verticalsynchronizing frequency, whereby moving image characteristics areimproved.

Furthermore, as a technique for improving moving image characteristicsof a liquid crystal display device, there is another driving techniquein which, as a backlight, a surface light source including a pluralityof LED (light-emitting diode) light sources or a plurality of EL lightsources is used, and each light source included in the surface lightsource is independently driven so as to perform intermittent lighting inone frame period. As the surface light source, three or more kinds ofLEDs may be used, or a white-light-emitting LED may be used. Since aplurality of LEDs can be controlled independently, the timing at whichthe LEDs emit light can be synchronized with the timing at which opticalmodulation of a liquid crystal layer is switched. In this drivingtechnique, part of LEDs can be turned off. Therefore, especially in thecase of displaying an image in which the proportion of a black imagearea in one screen is high, a liquid crystal display device can bedriven with low power consumption.

When combined with any of these driving techniques, a liquid crystaldisplay device can have better display characteristics such as movingimage characteristics than conventional liquid crystal display devices.

Since the thin film transistor is easily broken due to staticelectricity or the like, the protective circuit is preferably providedover the same substrate as the pixel portion and the driver circuitportion. The protective circuit is preferably formed using a non-linearelement including an oxide semiconductor layer. For example, aprotective circuit is provided between the pixel portion, and a scanline input terminal and a signal line input terminal. In thisembodiment, a plurality of protective circuits are provided so that thepixel transistor and the like are not broken when surge voltage due tostatic electricity or the like is applied to the scan line, the signalline, or a capacitor bus line. Accordingly, the protective circuit has astructure for releasing charge to a common wiring when surge voltage isapplied to the protective circuit. The protective circuit includesnon-linear elements which are arranged in parallel between the scan lineand the common wiring. Each of the non-linear elements includes atwo-terminal element such as a diode or a three-terminal element such asa transistor. For example, the non-linear element can be formed throughthe same steps as the thin film transistor of the pixel portion. Forexample, characteristics similar to those of a diode can be achieved byconnecting a gate terminal to a drain terminal.

Further, for the liquid crystal display module, a twisted nematic (TN)mode, an in-plane-switching (IPS) mode, a fringe field switching (FFS)mode, an axially symmetric aligned micro-cell (ASM) mode, an opticalcompensated birefringence (OCB) mode, a ferroelectric liquid crystal(FLC) mode, an antiferroelectric liquid crystal (AFLC) mode, or the likecan be used.

There is no particular limitation in the semiconductor device disclosedin this specification, and a liquid crystal display device including aTN liquid crystal, an OCB liquid crystal, an STN liquid crystal, a VAliquid crystal, an ECB liquid crystal, a GH liquid crystal, a polymerdispersed liquid crystal, a discotic liquid crystal, or the like can beused. In particular, a normally black liquid crystal panel such as atransmissive liquid crystal display device utilizing a verticalalignment (VA) mode is preferable. Some examples are given as a verticalalignment mode. For example, an MVA (multi-domain vertical alignment)mode, a PVA (patterned vertical alignment) mode, an ASV mode, or thelike can be employed.

An embodiment of the present invention can also be applied to a VAliquid crystal display device. The VA liquid crystal display device hasa kind of form in which alignment of liquid crystal molecules in aliquid crystal display panel is controlled. In the VA liquid crystaldisplay device, liquid crystal molecules are aligned in a verticaldirection with respect to a panel surface when no voltage is applied. Amethod called multi-domain or multi-domain design in which a pixel isdivided into some regions (subpixels) and molecules are aligned indifferent directions in their respective regions can be used.

The photodetector 4100 detects the illuminance in the vicinity of theliquid crystal display device, and the light-emission luminance from thebacklight can be adjusted, whereby the visibility can be increased andlower power consumption can be achieved.

If the photodetector described in Embodiment 1 is provided in the pixelportion 4002, the device can be used as an optical touch sensor.

This embodiment can be implemented in appropriate combination with anyof the other embodiments.

Embodiment 11

In this embodiment, an example of an active matrix light-emittingdisplay device will be described. Specifically, an example of alight-emitting display device including a light-emitting elementutilizing electroluminescence will be described.

Light-emitting elements utilizing electroluminescence are classifiedaccording to whether a light-emitting material is an organic compound oran inorganic compound. In general, the former is referred to as anorganic EL element, and the latter is referred to as an inorganic ELelement.

In an organic EL element, by application of voltage to a light-emittingelement, electrons and holes are separately injected from a pair ofelectrodes into a layer containing a light-emitting organic compound,and current flows. Then, the carriers (electrons and holes) recombineand light emission is caused. Owing to such a mechanism, thislight-emitting element is referred to as a current-excitationlight-emitting element.

The inorganic EL elements are classified according to their elementstructures into a dispersion-type inorganic EL element and a thin-filminorganic EL element. A dispersion-type inorganic EL element has alight-emitting layer where particles of a light-emitting material aredispersed in a binder, and its light emission mechanism isdonor-acceptor recombination type light emission which utilizes a donorlevel and an acceptor level. A thin-film inorganic EL element has astructure where a light-emitting layer is sandwiched between dielectriclayers, which are further sandwiched between electrodes, and its lightemission mechanism is localized type light emission that utilizesinner-shell electron transition of metal ions. Note that description ismade in this embodiment using an organic EL element as a light-emittingelement.

FIG. 13 illustrates an example of a pixel configuration to which digitaltime grayscale driving can be applied as an example of the semiconductordevice.

The configuration and operation of a pixel to which digital timegrayscale driving can be applied will be described. An example isdescribed in this embodiment in which one pixel includes two n-channeltransistors using an oxide semiconductor layer in a channel region.

A pixel 6400 includes a switching transistor 6401, a transistor 6402 fordriving a light-emitting element, a light-emitting element 6404, and acapacitor 6403. In the switching transistor 6401, a gate thereof isconnected to a scan line 6406, a first electrode thereof (one of sourceand drain electrodes) is connected to a signal line 6405, and a secondelectrode thereof (the other of the source and drain electrodes) isconnected to a gate of the transistor 6402 for driving a light-emittingelement. In the transistor 6402 for driving a light-emitting element,the gate thereof is connected to a power supply line 6407 through thecapacitor 6403, a first electrode thereof is connected to the powersupply line 6407, and a second electrode thereof is connected to a firstelectrode (pixel electrode) of the light-emitting element 6404. A secondelectrode of the light-emitting element 6404 corresponds to a commonelectrode 6408. The common electrode 6408 is electrically connected to acommon potential line provided over the same substrate.

Note that the second electrode (common electrode 6408) of thelight-emitting element 6404 is set to a low power supply potential. Notethat the low power supply potential is a potential satisfying the lowpower supply potential<a high power supply potential with reference tothe high power supply potential that is set on the power supply line6407. As the low power supply potential, GND, 0 V, or the like may beemployed, for example. The difference between the high power supplypotential and the low power supply potential is applied to thelight-emitting element 6404 so that current flows through thelight-emitting element 6404, whereby the light-emitting element 6404emits light. Thus, each potential is set so that the difference betweenthe high power supply potential and the low power supply potential isgreater than or equal to a forward threshold voltage of thelight-emitting element 6404.

When the gate capacitance of the transistor 6402 for driving alight-emitting element is used as a substitute for the capacitor 6403,the capacitor 6403 can be omitted. The gate capacitance of thetransistor 6402 for driving a light-emitting element may be formedbetween the channel region and the gate electrode.

In the case of using a voltage-input voltage driving method, a videosignal is inputted to the gate of the transistor 6402 for driving alight-emitting element so that the transistor 6402 for driving alight-emitting element is in either of two states of being sufficientlyturned on and turned off. That is, the transistor 6402 for driving alight-emitting element operates in a linear region, and thus a voltagehigher than the voltage of the power supply line 6407 is applied to thegate of the transistor 6402 for driving a light-emitting element. Notethat a voltage higher than or equal to the following is applied to thesignal line 6405: power supply line voltage+V_(th) of the transistor6402 for driving a light-emitting element.

In the case of performing analog grayscale driving instead of digitaltime grayscale driving, the same pixel configuration as FIG. 13 can beemployed by changing signal input.

In the case of performing analog grayscale driving, voltage higher thanor equal to the following is applied to the gate of the transistor 6402for driving a light-emitting element: forward voltage of thelight-emitting element 6404+V_(th) of the transistor 6402 for driving alight-emitting element. The forward voltage of the light-emittingelement 6404 refers to voltage to obtain a desired luminance, andincludes at least forward threshold voltage. By input of a video signalwhich enables the transistor 6402 for driving a light-emitting elementto operate in a saturation region, it is possible to feed current to thelight-emitting element 6404. In order that the transistor 6402 fordriving a light-emitting element can operate in the saturation region,the potential of the power supply line 6407 is set higher than a gatepotential of the transistor 6402 for driving a light-emitting element.When an analog video signal is used, it is possible to feed current tothe light-emitting element 6404 in accordance with the video signal andperform analog grayscale driving.

Next, an example of a pixel configuration which is different from thatof FIG. 13 will be described with reference to FIG. 14. FIG. 14illustrates an example of a pixel configuration to which a currentmirror circuit is applied. The example here is an example in which fourn-channel transistors in which an oxide semiconductor layer is used fora channel region are used in one pixel.

A pixel 6510 includes a switching transistor 6511, a switchingtransistor 6512, a reference transistor 6513, a transistor 6502 fordriving a light-emitting element, a light-emitting element 6504, and acapacitor 6503. Gates of the switching transistor 6511 and the switchingtransistor 6512 are connected to a scan line 6506. A first electrode(one of a source electrode and a drain electrode) of the switchingtransistor 6511 is connected to a signal line 6505, and a secondelectrode (the other of the source electrode and the drain electrode)thereof is connected to gates of the reference transistor 6513 and thetransistor 6502 for driving a light-emitting element. A first electrodeof the switching transistor 6512 is connected to the signal line 6505,and a second electrode thereof is connected to a first electrode of thereference transistor 6513.

A first electrode of the transistor 6502 for driving a light-emittingelement is connected to a power supply line 6507, and the gate thereofis connected to a first electrode (pixel electrode) of thelight-emitting element 6504 through the capacitor 6503. Note thatalthough the capacitor 6503 is connected to the first electrode of thelight-emitting element 6504 in FIG. 14, the capacitor 6503 may beconnected to not the first electrode of the light-emitting element 6504but the power supply line 6507 or an electrode having a fixed potentialsuch as a common electrode 6508. A structure may be employed in whichgates of the switching transistor 6511 and the switching transistor 6512are connected to a scan line different from the scan line 6506.

In addition, second electrodes of the reference transistor 6513 and thetransistor 6502 for driving a light-emitting element are connected tothe first electrode (pixel electrode) of the light-emitting element6504, and a second electrode of the light-emitting element 6504 isconnected to the common electrode 6508. The common electrode 6508 iselectrically connected to a common potential line that is formed overthe same substrate.

The common electrode 6508 is set to a low power supply potential. Notethat the low power supply potential is a potential lower than a highpower supply potential which is set to the power supply line 6507. Asthe low power supply potential, GND, 0 V, or the like may be employed,for example. The light-emitting element 6504 is made to emit light byflow of current I_(out), which is supplied from the power supply line6507, to the light-emitting element 6504 through the transistor 6502 fordriving a light-emitting element. Thus, each potential is set so thatthe difference between the high power supply potential and the low powersupply potential is greater than or equal to a forward threshold voltageof the light-emitting element 6504.

When the gate capacitance of the transistor 6502 for driving alight-emitting element is used as a substitute for the capacitor 6503,the capacitor 6503 can be omitted. The gate capacitance of thetransistor 6502 for driving a light-emitting element may be formedbetween a channel region and a gate electrode.

First, a potential of the scan line 6506 is set to such a potential asto turn on the switching transistor 6511 and the switching transistor6512, so that the first electrode and the second electrode of theswitching transistor 6511 are brought into electrical conduction and thefirst electrode and the second electrode of the switching transistor6512 are also brought into electrical conduction, whereby currentI_(data), is supplied from the signal line 6505 to the pixel circuit.Current I_(data) is supplied through the switching transistor 6511 tothe capacitor 6503, and the capacitor 6503 is charged. The potential ofthe capacitor 6503 is increased by the charging, and when the potentialbecomes higher than Vth of the reference transistor 6513, the referencetransistor 6513 is turned on. Then, current I_(data) flows through theswitching transistor 6512, the reference transistor 6513, and thelight-emitting element 6504 to the common electrode 6508.

The increase in potential of the capacitor 6503 continues until the timewhen the drain current of the reference transistor 6513 becomes equal toa current value of current Id. In other words, the increase in potentialof the capacitor 6503 stops when the flow of current I_(data) throughthe switching transistor 6511 stops.

Since the gates of the reference transistor 6513 and the transistor 6502for driving a light-emitting element are connected to each other, thegate of the transistor 6502 for driving a light-emitting element and thegate of the reference transistor 6513 have the same potential. If thereference transistor 6513 and the transistor 6502 for driving alight-emitting element have the same transistor characteristics and thesame ratio of the channel width W to the channel length L (W/L ratio),current I_(out) having an equal current value to current I_(data) issupplied from the power supply line 6507 to the light-emitting element6504 through the transistor 6502 for driving a light-emitting element.

Next, the potential of the scan line 6506 is set to such a potential asto turn off the switching transistor 6511 and the switching transistor6512, so that the switching transistor 6511 and the switching transistor6512 become in an off state and supply of current I_(data) stops.However, current I_(data) can continue to be supplied to thelight-emitting element 6504 by the potential held in the capacitor 6503.

Note that current I_(data) can be made larger or smaller than currentI_(data) by devising transistor characteristics and the relation betweenthe channel width W and the channel length L of the reference transistor6513 and the transistor 6502 for driving a light-emitting element. Forexample, if a transistor which has the same transistor characteristicsand the same channel length L as the reference transistor 6513 and hashalf the channel width W of the reference transistor 6513 is used as thetransistor 6502 for driving a light-emitting element, current I_(data)can become half of current I_(data).

Since a transistor including an oxide semiconductor layer used in thisembodiment has an extremely small off-state current, the potential ofthe capacitor 6503 can be held easily and the size of the capacitor 6503can be made small. Further, a phenomenon of faint light emission due tooff-state current caused when the light-emitting element 6504 is in anon-emitting state without being supplied with a current can beprevented.

Note that the pixel configuration is not limited to those illustrated inFIG. 13 and FIG. 14. For example, the pixels illustrated in FIG. 13 andFIG. 14 may further include a switch, a resistor, a capacitor, atransistor, a logic circuit, or the like.

Next, structures of the light-emitting element will be described withreference to FIGS. 15A to 15C. A cross-sectional structure of a pixelwill be described by taking an n-channel thin film transistor fordriving a light-emitting element as an example. A transistor 7011 fordriving a light-emitting element, a transistor 7021 for driving alight-emitting element, and a transistor 7001 for driving alight-emitting element which are used for semiconductor devicesillustrated in FIGS. 15A, 15B, and 15C, respectively, can bemanufactured in a manner similar to that of the thin film transistordescribed in any of the above embodiments, and examples of using a thinfilm transistor including an oxide semiconductor layer will bedescribed.

In order to extract light emitted from the light-emitting element, atleast one of the anode and the cathode is required to transmit light. Athin film transistor and a light-emitting element are formed over asubstrate. A light-emitting element can have a top emission structure inwhich light is extracted through the surface opposite to the substrate,a bottom emission structure in which light is extracted through thesurface on the substrate side, or a dual emission structure in whichlight is extracted through the surface opposite to the substrate and thesurface on the substrate side. The pixel configuration can be applied toa light-emitting element having any of these emission structures.

A light-emitting element having a bottom emission structure will bedescribed with reference to FIG. 15A.

FIG. 15A is a cross-sectional view of a pixel in the case where thetransistor 7011 for driving a light-emitting element is of an n-type andlight is emitted from a light-emitting element 7012 to a first electrode7013 side. In FIG. 15A, the first electrode 7013 of the light-emittingelement 7012 is formed over a light-transmitting conductive film 7017which is electrically connected to a drain electrode layer of thetransistor 7011 for driving a light-emitting element, and an EL layer7014 and a second electrode 7015 are stacked in that order over thefirst electrode 7013.

As the light-transmitting conductive film 7017, a light-transmittingconductive film such as a film of indium oxide containing tungstenoxide, indium zinc oxide containing tungsten oxide, indium oxidecontaining titanium oxide, indium tin oxide containing titanium oxide,indium tin oxide, indium zinc oxide, or indium tin oxide to whichsilicon oxide is added can be used.

The first electrode 7013 of the light-emitting element can be formedusing various materials. For example, in the case where the firstelectrode 7013 is used as a cathode, a material having a low workfunction, for example, an alkali metal such as Li or Cs, analkaline-earth metal such as Mg, Ca, or Sr, an alloy containing any ofthese (Mg: Ag, Al: Li, or the like), a rare-earth metal such as Yb orEr, or the like, is preferably used. In FIG. 15A, the thickness of thefirst electrode 7013 is such that the first electrode transmits light(preferably, approximately 5 nm to 30 nm). For example, an aluminum filmhaving a thickness of 20 nm is used for the first electrode 7013.

Note that the light-transmitting conductive film and the aluminum filmmay be stacked and then selectively etched, so that thelight-transmitting conductive film 7017 and the first electrode 7013 maybe formed. In this case, the etching can be performed using the samemask, which is preferable.

The peripheral portion of the first electrode 7013 is covered with apartition 7019. The partition 7019 is formed using an organic resin filmsuch as polyimide, an acrylic resin, polyamide, or an epoxy resin, aninorganic insulating film, or organic polysiloxane. It is particularlypreferable that the partition 7019 be formed using a photosensitiveresin material to have an opening over the first electrode 7013 so thata sidewall of the opening is formed as an inclined surface withcontinuous curvature. In the case where a photosensitive resin materialis used for the partition 7019, a step of forming a resist mask can beomitted.

The EL layer 7014 formed over the first electrode 7013 and the partition7019 may be formed using a single layer or a plurality of layers stackedas long as it includes at least a light-emitting layer. When the ELlayer 7014 is formed using a plurality of layers, the EL layer 7014 isformed by stacking an electron-injection layer, an electron-transportlayer, a light-emitting layer, a hole-transport layer, and ahole-injection layer in that order over the first electrode 7013functioning as a cathode. Note that not all of these layers need to beprovided.

The stacking order is not limited to the above stacking order. The firstelectrode 7013 may function as an anode, and a hole-injection layer, ahole-transport layer, a light-emitting layer, an electron-transportlayer, and an electron-injection layer may be stacked in that order overthe first electrode 7013. However, when power consumption is compared,it is preferable that the first electrode 7013 function as a cathode andan electron-injection layer, an electron-transport layer, alight-emitting layer, a hole-transport layer, and a hole-injection layerbe stacked in that order over the first electrode 7013, because anincrease in voltage in the driver circuit portion can be suppressed andpower consumption can be reduced.

As the second electrode 7015 formed over the EL layer 7014, variousmaterials can be employed. For example, in the case where the secondelectrode 7015 is used as an anode, a material having a high workfunction such as ZrN, Ti, W, Ni, Pt, or Cr; or a light-transmittingconductive material such as ITO, IZO, or ZnO is preferably used. Alight-blocking film 7016 is formed over the second electrode 7015. Asthe light-blocking film 7016, a metal which blocks light, a metal whichreflects light, or the like is used. In this embodiment, an ITO film isused for the second electrode 7015, and a Ti film is used for thelight-blocking film 7016.

The light-emitting element 7012 corresponds to a region where the ELlayer 7014 including a light-transmitting layer is sandwiched betweenthe first electrode 7013 and the second electrode 7015. In the case ofthe element structure illustrated in FIG. 15A, light is emitted from thelight-emitting element 7012 to the first electrode 7013 side asindicated by an arrow. Light emitted from the light-emitting element7012 passes through a color filter layer 7033, and can be emittedthrough the substrate.

The color filter layer 7033 is formed by a droplet discharge method suchas an ink-jet method, a printing method, an etching method with the useof a photolithography technique, or the like.

The color filter layer 7033 is covered with an overcoat layer 7034, andalso covered with a protective insulating layer 7035. Note that theovercoat layer 7034 with a thin thickness is illustrated in FIG. 15A;however, the overcoat layer 7034 has a function to planarize a surfacewith unevenness due to the color filter layer 7033.

A contact hole which is formed in the protective insulating layer 7035,an overcoat layer 7034, a color filter layer 7033, a planarizinginsulating film 7036, an insulating layer 7032, and an insulating layer7031 and which reaches the drain electrode layer is provided in aportion which overlaps with the partition 7019.

Next, a light-emitting element having a dual emission structure will bedescribed with reference to FIG. 15B.

In FIG. 15B, a first electrode 7023 of a light-emitting element 7022 isformed over a light-transmitting conductive film 7027 which iselectrically connected to a drain electrode layer of the transistor 7021for driving a light-emitting element, and an EL layer 7024 and a secondelectrode 7025 are stacked in that order over the first electrode 7023.

For the light-transmitting conductive film 7027, a light-transmittingconductive film of indium oxide containing tungsten oxide, indium zincoxide containing tungsten oxide, indium oxide containing titanium oxide,indium tin oxide containing titanium oxide, indium tin oxide, indiumzinc oxide, indium tin oxide to which silicon oxide is added, or thelike can be used.

The first electrode 7023 can be formed using various materials. Forexample, in the case where the first electrode 7023 is used as acathode, a material having a low work function, specifically, an alkalimetal such as Li or Cs; an alkaline-earth metal such as Mg, Ca, or Sr;an alloy containing any of these (Mg: Ag, Al: Li, or the like); arare-earth metal such as Yb or Er; or the like is preferable. In thisembodiment, the first electrode 7023 is used as a cathode and the firstelectrode 7023 is formed to a thickness such that the first electrode7023 can transmit light (preferably, approximately 5 nm to 30 nm). Forexample, a 20-nm-thick aluminum film is used as the cathode.

Note that the light-transmitting conductive film and the aluminum filmmay be stacked and then selectively etched, so that thelight-transmitting conductive film 7027 and the first electrode 7023 maybe formed. In that case, etching can be performed with the use of thesame mask, which is preferable.

The periphery of the first electrode 7023 is covered with a partition7029. The partition 7029 is formed using an organic resin film such aspolyimide, an acrylic resin, polyamide, or an epoxy resin; an inorganicinsulating film; or organic polysiloxane. It is particularly preferablethat the partition 7029 be formed using a photosensitive resin materialto have an opening over the first electrode 7023 so that a sidewall ofthe opening is formed as an inclined surface with continuous curvature.In the case where a photosensitive resin material is used for thepartition 7029, a step of forming a resist mask can be omitted.

The EL layer 7024 formed over the first electrode 7023 and the partition7029 may be formed using either a single layer or a plurality of layersstacked as long as it includes at least a light-emitting layer. When theEL layer 7024 is formed using a plurality of layers, the EL layer 7024is formed by stacking an electron-injection layer, an electron-transportlayer, a light-emitting layer, a hole-transport layer, and ahole-injection layer in that order over the first electrode 7023functioning as a cathode. Note that not all of these layers need to beprovided.

The stacking order is not limited to the above. The first electrode 7023may be used as an anode, and a hole-injection layer, a hole-transportlayer, a light-emitting layer, an electron-transport layer, and anelectron-injection layer may be stacked in that order over the anode.However, for lower power consumption, it is preferable that the firstelectrode 7023 be used as a cathode and an electron-injection layer, anelectron-transport layer, a light-emitting layer, a hole-transportlayer, and a hole-injection layer be stacked in this order over thecathode.

In addition, the second electrode 7025 formed over the EL layer 7024 canbe formed using a variety of materials. For example, when the secondelectrode 7025 is used as an anode, a material with a high work functionor a transparent conductive material such as ITO, IZO, or ZnO ispreferable. In this embodiment, the second electrode 7025 is formedusing an ITO film including silicon oxide and is used as an anode.

The light-emitting element 7022 corresponds to a region where the ELlayer 7024 including a light-emitting layer is sandwiched between thefirst electrode 7023 and the second electrode 7025. In the case of theelement structure illustrated in FIG. 15B, light emitted from thelight-emitting element 7022 is emitted to both the second electrode 7025side and the first electrode 7023 side as indicated by arrows.

Note that an example in which a light-transmitting conductive film isused as a gate electrode layer and a light-transmitting thin film isused as source and drain electrode layers is illustrated in FIG. 15B.Light emitted from the light-emitting element 7022 to the firstelectrode 7023 side passes through a color filter layer 7043, and can beextracted through the substrate.

The color filter layer 7043 is formed by a droplet discharge method suchas an ink-jet method, a printing method, an etching method with the useof a photolithography technique, or the like.

The color filter layer 7043 is covered with an overcoat layer 7044, andalso covered with a protective insulating layer 7045.

A contact hole which is formed in the protective insulating layer 7045,the overcoat layer 7044, the color filter layer 7043, a planarizationinsulating layer 7046, an insulating layer 7042, and an insulating layer7041 and which reaches the drain electrode layer is provided so as to beoverlapped with the partition 7029.

Note that when a light-emitting element having a dual emission structureis used and full color display is performed on both display surfaces,light from the second electrode 7025 side does not pass through thecolor filter layer 7043; therefore, a sealing substrate provided withanother color filter layer is preferably provided over the secondelectrode 7025.

Next, a light-emitting element having a top emission structure isdescribed with reference to FIG. 15C.

FIG. 15C is a cross-sectional view of a pixel in the case where thetransistor 7001 for driving a light-emitting element is of n-type andlight is emitted from a light-emitting element 7002 to a secondelectrode 7005 side. In FIG. 15C, a drain electrode layer of thetransistor 7001 for driving a light-emitting element and a firstelectrode 7003 are in contact with each other, and the transistor 7001for driving a light-emitting element and the first electrode 7003 of thelight-emitting element 7002 are electrically connected to each other. AnEL layer 7004 and a second electrode 7005 are stacked in this order overthe first electrode 7003.

The first electrode 7013 can be formed using a variety of materials. Forexample, in the case where the first electrode 7013 is used as acathode, a material having a low work function, for example, an alkalimetal such as Li or Cs, an alkaline-earth metal such as Mg, Ca, or Sr,an alloy containing any of these (Mg: Ag, Al: Li, or the like), arare-earth metal such as Yb or Er, or the like, is preferably used.

The periphery of the first electrode 7003 is covered with a partition7009. The partition 7009 is formed using an organic resin film such aspolyimide, an acrylic resin, polyamide, or an epoxy resin; an inorganicinsulating film; or organic polysiloxane. It is particularly preferablethat the partition 7009 be formed using a photosensitive resin materialto have an opening over the first electrode 7003 so that a sidewall ofthe opening is inclined with continuous curvature. When the partition7009 is formed using a photosensitive resin material, a step of forminga resist mask can be omitted.

The EL layer 7004 formed over the first electrode 7003 and the partition7009 may be formed using either a single layer or a plurality of layersstacked as long as it includes at least a light-emitting layer. When theEL layer 7004 is formed using a plurality of layers, the EL layer 7004is formed by stacking an electron-injection layer, an electron-transportlayer, a light-emitting layer, a hole-transport layer, and ahole-injection layer in that order over the first electrode 7003 used asa cathode. Note that not all of these layers need to be provided.

The stacking order is not limited to the above stacking order, and ahole-injection layer, a hole-transport layer, a light-emitting layer, anelectron-transport layer, and an electron-injection layer may be stackedin that order over the first electrode 7003 used as an anode.

In FIG. 15C, a hole-injection layer, a hole-transport layer, alight-emitting layer, an electron-transport layer, and anelectron-injection layer are stacked in that order over a stacked filmin which a Ti film, an aluminum film, and a Ti film are stacked in thatorder, and thereover, a stacked layer of a Mg:Ag alloy thin film and ITOis formed.

However, in the case where the transistor 7001 for driving alight-emitting element is of an n-type, it is preferable that anelectron-injection layer, an electron-transport layer, a light-emittinglayer, a hole-transport layer, and a hole-injection layer be stacked inthat order over the first electrode 7003, because an increase in voltagein the driver circuit can be suppressed and power consumption can bereduced.

The second electrode 7005 is formed using a light-transmittingconductive material through which light can pass, and for example, alight-transmitting conductive film of indium oxide containing tungstenoxide, indium zinc oxide containing tungsten oxide, indium oxidecontaining titanium oxide, indium tin oxide containing titanium oxide,indium tin oxide, indium zinc oxide, indium tin oxide to which siliconoxide is added, or the like can be used.

The light-emitting element 7002 corresponds to a region where the ELlayer 7004 is sandwiched between the first electrode 7003 and the secondelectrode 7005. In the case of the pixel illustrated in FIG. 15C, lightis emitted from the light-emitting element 7002 to the second electrode7005 side as indicated by an arrow.

In FIG. 15C, the drain electrode layer of the transistor 7001 fordriving a light-emitting element is electrically connected to the firstelectrode 7003 through a contact hole provided in a silicon oxide layer7051, a protective insulating layer 7052, a planarization insulatinglayer 7056, a planarization insulating layer 7053, and an insulatinglayer 7055.

The partition 7009 is provided in order to insulate the first electrode7003 from a first electrode of an adjacent pixel. The partition 7009 isformed using an organic resin film of polyimide, an acrylic resin,polyamide, an epoxy resin, or the like; an inorganic insulating film; ororganic polysiloxane. It is particularly preferable that the partition7009 be formed using a photosensitive resin material to have the openingover the first electrode 7003 so that the sidewall of the opening isformed as a tilted surface with continuous curvature. When the partition7009 is formed using a photosensitive resin material, a step of forminga resist mask can be omitted.

In the structure of FIG. 15C, when full color display is performed, forexample, the light-emitting element 7002 is used as a greenlight-emitting element, one of adjacent light-emitting elements is usedas a red light-emitting element, and the other is used as a bluelight-emitting element. Alternatively, a light-emitting display devicecapable of full color display may be manufactured using four kinds oflight-emitting elements, which include white light-emitting elements inaddition to three kinds of light-emitting elements.

In the structure of FIG. 15C, a light-emitting display device capable offull color display may be manufactured in such a way that all of aplurality of light-emitting elements which is arranged is whitelight-emitting elements and a sealing substrate having a color filter orthe like is arranged on the light-emitting element 7002. A materialwhich exhibits light of a single color such as white can be formed andcombined with a color filter or a color conversion layer, whereby fullcolor display can be performed.

Note that the planarization insulating layers 7036, 7046, 7053, and 7056can be formed using a resin material such as polyimide, an acrylicresin, a benzocyclobutene-based resin, polyamide, or an epoxy resin. Inaddition to such resin materials, it is also possible to use alow-dielectric constant material (low-k material), a siloxane-basedresin, phosphosilicate glass (PSG), borophosphosilicate glass (BPSG), orthe like. Note that the planarization insulating layers 7036, 7046,7053, and 7056 may be formed by stacking a plurality of insulating filmsformed of these materials. There is no particular limitation on themethod for forming the planarization insulating layers 7036, 7046, 7053,and 7056, and the planarization insulating layers 7036, 7046, 7053, and7056 can be formed, depending on the material, by a method such as asputtering method, an SOG method, spin coating, dip coating, spraycoating, or a droplet discharge method (such as an ink-jet method,screen printing, offset printing, or the like), or a tool (equipment)such as a doctor knife, a roll coater, a curtain coater, or a knifecoater.

Any of the thin film transistors of the above embodiments can be used asappropriate as the transistor 7001 for driving a light-emitting element,the transistor 7011 for driving a light-emitting element, and thetransistor 7021 for driving a light-emitting element used forsemiconductor devices, and they can be formed using steps and materialssimilar to those for the thin film transistors of the above embodiments.In oxide semiconductor layers of the transistors 7001, 7011, and 7021for driving light-emitting elements, hydrogen or water is reduced.Therefore, the transistors 7001, 7011, and 7021 for drivinglight-emitting elements are highly reliable thin film transistors.

Needless to say, display of monochromatic light can also be performed.For example, a lighting device may be formed with the use of white lightemission, or an area-color light-emitting device may be formed with theuse of a single color light emission.

If necessary, an optical film such as a polarizing film including acircularly polarizing plate may be provided.

Although an organic EL element is described here as a light-emittingelement, an inorganic EL element can also be provided as alight-emitting element.

Note that the example is described in which a thin film transistor whichcontrols the driving of a light-emitting element (a transistor fordriving a light-emitting element) is electrically connected to thelight-emitting element; however, a structure may be employed in which atransistor for current control is connected between the transistor fordriving a light-emitting element and the light-emitting element.

Next, the appearance and a cross section of a light-emitting displaypanel (also referred to as a light-emitting panel) will be describedwith reference to FIGS. 16A and 16B. FIG. 16A is a plan view of a panelin which a thin film transistor and a light-emitting element are sealedbetween a first substrate and a second substrate with a sealant. FIG.16B is a cross-sectional view taken along line H-I of FIG. 16A.

A sealant 4505 is provided to surround a pixel portion 4502, a signalline driver circuit 4503 a, a signal line driver circuit 4503 b, a scanline driver circuit 4504 a, and a scan line driver circuit 4504 b, whichare provided over a first substrate 4501. In addition, a secondsubstrate 4506 is provided over the pixel portion 4502, the signal linedriver circuits 4503 a and 4503 b, and the scan line driver circuits4504 a and 4504 b. Accordingly, the pixel portion 4502, the signal linedriver circuits 4503 a and 4503 b, and the scan line driver circuits4504 a and 4504 b are sealed together with a filler 4507, by the firstsubstrate 4501, the sealant 4505, and the second substrate 4506. It ispreferable that a display device be thus packaged (sealed) with aprotective film (such as a bonding film or an ultraviolet curable resinfilm) or a cover material with high air-tightness and littledegasification so that the display device is not exposed to the outsideair.

A photodetector 4580 described in Embodiment 1 is provided in a regionthat is different from the region surrounded by the sealant 4505 overthe first substrate 4501. The photodetector 4580 may be formed over thefirst substrate 4501 at the same time as the pixel portion or may beformed over a different substrate and then mounted on the firstsubstrate 4501. Note that in the case of using a light-transmittingsubstrate as the first substrate 4501, the photodetector 4580 can have astructure of detecting light that enters from the substrate side, whilein the case of using a substrate that does not transmit visible light asthe first substrate 4501, a light-receiving portion of the photodetectorneeds to face in such a direction as not to be influenced by blocking oflight due to the substrate.

The pixel portion 4502, the signal line driver circuits 4503 a and 4503b, and the scan line driver circuits 4504 a and 4504 b formed over thefirst substrate 4501 each include a plurality of thin film transistors,and a thin film transistor 4510 included in the pixel portion 4502 and athin film transistor 4509 included in the signal line driver circuit4503 a are illustrated as an example in FIG. 16B.

Any of the thin film transistors of the above embodiments can be used asappropriate as the thin film transistors 4509 and 4510, and they can beformed using steps and materials similar to those for the thin filmtransistors of the above embodiments. Hydrogen or water is reduced inoxide semiconductor layers of the thin film transistors 4509 and 4510.

Note that the thin film transistor 4509 for a driver circuit has aconductive layer in a position which overlaps with the channel region ofthe oxide semiconductor layer in the thin film transistor. In thisembodiment, the thin film transistors 4509 and 4510 are n-channel thinfilm transistors.

A conductive layer 4540 is provided over a silicon oxide layer 4542 in aportion which overlaps with the channel region of the oxidesemiconductor layer of the thin film transistor 4509 for a drivercircuit. When the conductive layer 4540 is provided in a portion whichoverlaps with the channel region of the oxide semiconductor layer, theamount of shift in the threshold voltage of the thin film transistor4509 between before and after a BT (Bias Temperature) test can bereduced. The conductive layer 4540 may have a potential which is thesame as or different from that of the gate electrode layer of the thinfilm transistor 4509, and can function as a second gate electrode layer.The potential of the conductive layer 4540 may be GND, 0 V or in afloating state.

Further, the silicon oxide layer 4542 is formed to cover the oxidesemiconductor layer of the thin film transistor 4510. The source ordrain electrode layer of the thin film transistor 4510 is electricallyconnected to a wiring layer 4550 in an opening formed in the siliconoxide layer 4542 and an insulating layer 4551 which are formed over thethin film transistor. The wiring layer 4550 is formed in contact with afirst electrode 4517, and the thin film transistor 4510 and the firstelectrode 4517 are electrically connected to each other through thewiring layer 4550.

The silicon oxide layer 4542 may be formed using a material and a methodsimilar to those of the oxide insulating layers described in the otherembodiments.

A color filter layer 4545 is formed over the insulating layer 4551 so asto overlap with a light-emitting region of a light-emitting element4511.

Further, in order to reduce the surface roughness of the color filterlayer 4545, the color filter layer 4545 is covered with an overcoatlayer 4543 functioning as a planarization insulating film.

Further, an insulating layer 4544 is formed over the overcoat layer4543. The insulating layer 4544 may be formed in a manner similar tothat of the protective insulating layers described in the otherembodiments, and a silicon nitride film may be formed by a sputteringmethod, for example.

Reference numeral 4511 denotes a light-emitting element, and the firstelectrode 4517 that is a pixel electrode included in the light-emittingelement 4511 is electrically connected to a source electrode layer or adrain electrode layer of the thin film transistor 4510 through thewiring layer 4550. Note that a structure of the light-emitting element4511 is not limited to the illustrated structure, which includes thefirst electrode 4517, an electroluminescent layer 4512, and a secondelectrode 4513. The structure of the light-emitting element 4511 can bechanged as appropriate depending on the direction in which light isextracted from the light-emitting element 4511, or the like.

A partition 4520 is formed using an organic resin film, an inorganicinsulating film, or organic polysiloxane. It is particularly preferablethat the partition 4520 be formed of a photosensitive material to havean opening over the first electrode 4517 so that a sidewall of theopening is formed as an inclined surface with continuous curvature.

The electroluminescent layer 4512 may be formed using a single layer ora plurality of layers stacked.

A protective film may be formed over the second electrode 4513 and thepartition 4520 in order to prevent oxygen, hydrogen, moisture, carbondioxide, or the like from entering the light-emitting element 4511. Asthe protective film, a silicon nitride film, a silicon nitride oxidefilm, a DLC (Diamond-Like Carbon) film, or the like can be formed.

In addition, a variety of signals and potentials are supplied to thesignal line driver circuits 4503 a and 4503 b, the scan line drivercircuits 4504 a and 4504 b, or the pixel portion 4502 from an FPC 4518 aand an FPC 4518 b.

A connection terminal electrode 4515 is formed from the same conductivefilm as the first electrode 4517 included in the light-emitting element4511, and a terminal electrode 4516 is formed from the same conductivefilm as the source and drain electrode layers included in the thin filmtransistor 4509.

The connection terminal electrode 4515 is electrically connected to aterminal included in the FPC 4518 a through an anisotropic conductivefilm 4519.

The substrate located in the direction in which light is extracted fromthe light-emitting element 4511 needs to have a light-transmittingproperty. In that case, a light-transmitting material such as a glassplate, a plastic plate, a polyester film, or an acrylic resin film isused.

As the filler 4507, an ultraviolet curable resin or a thermosettingresin can be used, in addition to an inert gas such as nitrogen orargon. For example, PVC (polyvinyl chloride), acrylic, polyimide, anepoxy resin, a silicone resin, PVB (polyvinyl butyral), or EVA (acopolymer of ethylene with vinyl acetate) can be used. For example,nitrogen is used as the filler.

In addition, if needed, an optical film such as a polarizing plate, acircularly polarizing plate (including an elliptically polarizingplate), or a retardation plate (a quarter-wave plate or a half-waveplate) may be provided as appropriate on a light-emitting surface of thelight-emitting element. Further, the polarizing plate or the circularlypolarizing plate may be provided with an anti-reflection film. Forexample, anti-glare treatment by which reflected light can be diffusedby projections and depressions on the surface so as to reduce the glarecan be performed.

The sealant can be deposited using a screen printing method, an ink-jetapparatus, or a dispensing apparatus. As the sealant, typically, amaterial containing a visible light curable resin, an ultravioletcurable resin, or a thermosetting resin can be used. Further, a fillermay be contained.

The signal line driver circuits 4503 a and 4503 b and the scan linedriver circuits 4504 a and 4504 b may be mounted as driver circuitsformed using a single crystal semiconductor film or a polycrystallinesemiconductor film over a substrate separately prepared. Alternatively,only the signal line driver circuits or part thereof, or only the scanline driver circuits or part thereof may be separately formed andmounted. This embodiment is not limited to the structure illustrated inFIGS. 16A and 16B.

Through the above process, a light-emitting display device (displaypanel) as a semiconductor device can be manufactured.

The photodetector 4580 detects the illuminance in the vicinity of thelight-emitting display device, and the light-emission luminance can beadjusted, whereby the visibility can be increased and lower powerconsumption can be achieved.

If the photodetector described in Embodiment 1 is provided in the pixelportion 4502, the device can be used as an optical touch sensor.

This embodiment can be implemented in appropriate combination with anyof the other embodiments.

Embodiment 12

In this embodiment, an embodiment of a semiconductor device disclosed inthis specification will be described. Specifically, an example ofelectronic paper will be described as an embodiment of a semiconductordevice disclosed in this specification.

FIG. 17 illustrates active matrix electronic paper. A thin filmtransistor 581 used for the electronic paper can be any of the thin filmtransistors described in the above embodiments and can be manufacturedusing steps and materials similar to those of the thin film transistorsdescribed in the above embodiments. In this embodiment, an example inwhich the thin film transistor described in Embodiment 6 is used as thethin film transistor 581 will be described. Hydrogen or water is reducedin the oxide semiconductor layer of the thin film transistor 581.

The electronic paper in FIG. 17 is an example of a display device usinga twisting ball display system. The twisting ball display system refersto a method in which spherical particles each colored in black and whiteare used for a display element and arranged between a first electrodelayer and a second electrode layer which are electrode layers, and apotential difference is generated between the first electrode layer andthe second electrode layer to control orientation of the sphericalparticles, so that display is performed.

The thin film transistor 581 provided over a substrate 580 is a thinfilm transistor having a bottom-gate structure. A source or drainelectrode layer of the thin film transistor 581 is electricallyconnected to a first electrode layer 587 through an opening formed inthe silicon oxide layer 583, a protective insulating layer 584, and aninsulating layer 585.

Between the first electrode layer 587 and a second electrode layer 588,spherical particles 589 each having a black region 590 a, a white region590 b, and a cavity 594 which is filled with liquid around the blackregion 590 a and the white region 590 b are provided. A space around thespherical particles 589 is filled with a filler 595 such as a resin (seeFIG. 17). In this embodiment, the first electrode layer 587 correspondsto a pixel electrode, and the second electrode layer 588 provided on acounter substrate 596 corresponds to a common electrode.

Instead of the twisting ball, an electrophoretic element can also beused. A microcapsule having a diameter of approximately 10 μm to 200 μmin which transparent liquid, positively-charged white microparticles,and negatively-charged black microparticles are encapsulated is used. Inthe microcapsule which is provided between the first electrode layer andthe second electrode layer, when an electric field is applied by thefirst electrode layer and the second electrode layer, the whitemicroparticles and the black microparticles move to opposite sides fromeach other, so that white or black can be displayed. A display elementusing this principle is an electrophoretic display element and isgenerally called electronic paper. The electrophoretic display elementhas higher reflectance than a liquid crystal display element, and thusan auxiliary light is unnecessary, power consumption is low, and adisplay portion can be recognized even in a dim place. In addition, evenwhen power is not supplied to the display portion, an image which hasbeen displayed once can be maintained. Accordingly, a displayed imagecan be stored even if a semiconductor device having a display function(which may be referred to simply as a display device or a semiconductordevice provided with a display device) is distanced from an electricwave source.

The electronic paper of this embodiment is a reflective display device,in which display is performed by controlling voltage applied to thetwisting ball with a driver circuit.

Through the above-described process, electronic paper as a semiconductordevice can be manufactured.

If the photodetector described in Embodiment 1 is provided in thedisplay portion, the device can be used as an optical touch sensor.

This embodiment can be implemented in appropriate combination with anyof the other embodiments.

Embodiment 13

A semiconductor device disclosed in this specification can be applied toa variety of electronic appliances (including amusement machines).Examples of electronic appliances include television sets (also referredto as televisions or television receivers), monitors of computers or thelike, cameras such as digital cameras or digital video cameras, digitalphoto frames, cellular phones (also referred to as mobile phones ormobile phone sets), portable game consoles, portable informationterminals, audio reproducing devices, large-sized game machines such aspachinko machines, and the like.

FIG. 18A illustrates an example of a cellular phone. A cellular phone1600 is provided with a display portion 1602 incorporated in a housing1601, operation buttons 1603 a and 1603 b, an external connection port1604, a speaker 1605, a microphone 1606, and the like.

When the display portion 1602 of the cellular phone 1600 illustrated inFIG. 18A is touched with a finger or the like, data can be input intothe cellular phone 1600. Further, operations such as making calls andcomposing mails can be performed by touching the display portion 1602with a finger or the like.

There are mainly three screen modes of the display portion 1602. Thefirst mode is a display mode mainly for displaying an image. The secondmode is an input mode mainly for inputting data such as text. The thirdmode is a display-and-input mode in which two modes of the display modeand the input mode are combined.

For example, in the case of making a call or composing a mail, a textinput mode mainly for inputting text is selected in the display portion1602 so that text displayed on a screen can be input. In this case, itis preferable to display a keyboard or number buttons on almost all areaof the screen of the display portion 1602.

When a detection device including a sensor for detecting inclination,such as a gyroscope or an acceleration sensor, is provided inside thecellular phone 1600, display of the screen on the display portion 1602can be automatically switched by determining the direction of thecellular phone 1600 (whether the cellular phone 1600 is placedhorizontally or vertically for a landscape mode or a portrait mode).

The screen modes are switched by touching the display portion 1602 oroperating the operation buttons 1603 a and 1603 b of the housing 1601.Alternatively, the screen modes may be switched depending on the kind ofthe image displayed on the display portion 1602. For example, when asignal of an image displayed on the display portion is a signal ofmoving image data, the screen mode is switched to the display mode. Whenthe signal is a signal of text data, the screen mode is switched to theinput mode.

Further, in the input mode, when input by touching the display portion1602 is not performed for a certain period while a signal detected bythe optical sensor in the display portion 1602 is detected, the screenmode may be controlled so as to be switched from the input mode to thedisplay mode.

The display portion 1602 may function as an image sensor. For example,an image of the palm print, the fingerprint, or the like is taken bytouching the display portion 1602 with the palm or the finger, wherebypersonal authentication can be performed. Further, by providing abacklight or sensing light source emitting a near-infrared light for thedisplay portion, an image of a finger vein, a palm vein, or the like canbe taken.

Any of the semiconductor devices described in the above embodiments canbe applied to the display portion 1602. For example, a plurality of thinfilm transistors described in the above embodiments can be disposed asswitching elements in pixels.

FIG. 18B also illustrates an example of a mobile phone. A portableinformation terminal such as the one illustrated in FIG. 18B can have aplurality of functions. For example, in addition to a telephonefunction, such a portable information terminal can have a function ofprocessing a variety of pieces of data by incorporating a computer.

The portable information terminal illustrated in FIG. 18B has a housing1800 and a housing 1801. The housing 1801 includes a display panel 1802,a speaker 1803, a microphone 1804, a pointing device 1806, a camera lens1807, an external connection terminal 1808, and the like. The housing1800 includes a keyboard 1810, an external memory slot 1811, and thelike. In addition, an antenna is incorporated in the housing 1801.

The display panel 1802 is provided with a touch panel. A plurality ofoperation keys 1805 which is displayed as images is illustrated bydashed lines in FIG. 18B.

Further, in addition to the above structure, a contactless IC chip, asmall memory device, or the like may be incorporated.

Any of the semiconductor devices described in the above embodiments canbe used for the display panel 1802 and the direction of display ischanged appropriately depending on an application mode. Further, thecamera lens 1807 is provided on the same surface as the display panel1802, and thus a videophone is realized. The speaker 1803 and themicrophone 1804 can be used for videophone calls, recording, and playingsound, etc. as well as voice calls. Moreover, the housings 1800 and 1801in a state where they are developed as illustrated in FIG. 18B can shiftso that one is lapped over the other by sliding; therefore, the size ofthe portable information terminal can be reduced, which makes theportable information terminal suitable for being carried.

The external connection terminal 1808 can be connected to an AC adapterand various types of cables such as a USB cable, and charging and datacommunication with a personal computer are possible. Moreover, a storagemedium can be inserted into the external memory slot 1811 so that alarge amount of data can be stored and can be moved.

Further, in addition to the above functions, an infrared communicationfunction, a television reception function, or the like may be provided.

FIG. 19A illustrates an example of a television set. In a television set9600, a display portion 9603 is incorporated in a housing 9601. Imagescan be displayed on the display portion 9603. Here, the housing 9601 issupported by a stand 9605.

The television set 9600 can be operated with an operation switch of thehousing 9601 or a separate remote controller 9610. Channels and volumecan be controlled with an operation key 9609 of the remote controller9610 so that an image displayed on the display portion 9603 can becontrolled. Furthermore, the remote controller 9610 may be provided witha display portion 9607 for displaying data output from the remotecontroller 9610.

Note that the television set 9600 is provided with a receiver, a modem,and the like. With the receiver, a general television broadcast can bereceived. Furthermore, when the television set 9600 is connected to acommunication network by wired or wireless connection via the modem,one-way (from a transmitter to a receiver) or two-way (between atransmitter and a receiver, between receivers, or the like) datacommunication can be performed.

Any of the semiconductor devices described in the above embodiments canbe applied to the display portion 9603. For example, a plurality of thinfilm transistors described in the above embodiments can be disposed asswitching elements in pixels.

FIG. 19B illustrates an example of a digital photo frame. For example,in a digital photo frame 9700, a display portion 9703 is incorporated ina housing 9701. Various images can be displayed on the display portion9703. For example, the display portion 9703 can display data of an imageshot by a digital camera or the like to function as a normal photoframe.

Any of the semiconductor devices described in the above embodiments canbe applied to the display portion 9703. For example, a plurality of thinfilm transistors described in the above embodiments can be disposed asswitching elements in pixels.

Note that the digital photo frame 9700 is provided with an operationportion, an external connection terminal (a USB terminal, a terminalthat can be connected to various cables such as a USB cable, or thelike), a recording medium insertion portion, and the like. Although theymay be provided on the same surface as the display portion, they arepreferably provided on the side surface or the back surface for thedesign of the digital photo frame 9700. For example, a memory thatstores data of an image shot by a digital camera is inserted into therecording medium insertion portion of the digital photo frame, wherebythe image data can be transferred and displayed on the display portion9703.

The digital photo frame 9700 may have a configuration capable ofwirelessly transmitting and receiving data. Through wirelesscommunication, desired image data can be transferred to be displayed.

FIG. 20 illustrates an example of a portable amusement machine. Theportable amusement machine illustrated in FIG. 20 is formed of twohousings: a housing 9881 and a housing 9891. The housings 9881 and 9891are connected with a connection portion 9893 so as to be opened andclosed. A display portion 9882 and a display portion 9883 areincorporated in the housing 9881 and the housing 9891, respectively.

Any of the semiconductor devices described in the above embodiments canbe applied to the display portion 9883. For example, a plurality of thinfilm transistors described in the above embodiments can be disposed asswitching elements in pixels.

In addition, the portable amusement machine illustrated in FIG. 20includes a speaker portion 9884, a recording medium insertion portion9886, an LED lamp 9890, an input unit (an operation key 9885, aconnection terminal 9887, a sensor 9888 (a sensor having a function ofmeasuring force, displacement, position, speed, acceleration, angularvelocity, rotational frequency, distance, light, liquid, magnetism,temperature, chemical substance, sound, time, hardness, electric field,current, voltage, electric power, radiation, flow rate, humidity,gradient, oscillation, odor, or infrared rays), or a microphone 9889),and the like. It is needless to say that the structure of the portableamusement machine is not limited to the above and other structuresprovided with at least a thin film transistor disclosed in thisspecification can be employed. The portable amusement machine mayinclude other accessory equipment as appropriate. The portable amusementmachine illustrated in FIG. 20 has a function of reading a program ordata stored in a recording medium to display it on the display portion,and a function of sharing information with another portable amusementmachine by wireless communication. The portable amusement machineillustrated in FIG. 20 can have various functions without limitation tothe above.

Embodiment 14

A semiconductor device disclosed in this specification can be applied toelectronic paper. Electronic paper can be used for electronic appliancesof a variety of fields as long as they can display data. For example,electronic paper can be applied to display portions of an e-book(electronic book) reader, a poster, an advertisement in a vehicle suchas a train, various cards such as a credit card, and the like. Anexample of the electronic appliances is illustrated in FIG. 21.

FIG. 21 illustrates an example of an electronic book reader. Forexample, an electronic book reader 2700 includes two housings, a housing2701 and a housing 2703. The housing 2701 and the housing 2703 arecombined with a hinge 2711 so that the electronic book reader 2700 canbe opened and closed with the hinge 2711 as an axis. With such astructure, the electronic book reader 2700 can operate like a paperbook.

A display portion 2705 and a display portion 2707 are incorporated inthe housing 2701 and the housing 2703, respectively. The display portion2705 and the display portion 2707 may display one image or differentimages. In the case where the display portion 2705 and the displayportion 2707 display different images, for example, text can bedisplayed on a display portion on the right side (the display portion2705 in FIG. 21) and images can be displayed on a display portion on theleft side (the display portion 2707 in FIG. 21).

FIG. 21 illustrates an example in which the housing 2701 is providedwith an operation portion and the like. For example, the housing 2701 isprovided with a power switch 2721, an operation key 2723, a speaker2725, and the like. With the operation key 2723, pages can be turned.Note that a keyboard, a pointing device, and the like may be provided onthe same surface as the display portion of the housing. Furthermore, anexternal connection terminal (an earphone terminal, a USB terminal, aterminal that can be connected to an AC adapter or various cables suchas a USB cable, or the like), a recording medium insertion portion, orthe like may be provided on the back surface or the side surface of thehousing. Moreover, the electronic book reader 2700 may have a functionof an electronic dictionary.

Further, the electronic book reader 2700 may send and receiveinformation wirelessly. Through wireless communication, desired bookdata or the like can be purchased and downloaded from an electronic bookserver.

This embodiment can be implemented in appropriate combination with anyof the other embodiments.

This application is based on Japanese Patent Application serial no.2009-242853 filed with Japan Patent Office on Oct. 21, 2009, the entirecontents of which are hereby incorporated by reference.

EXPLANATION OF REFERENCE

300: substrate, 302: gate insulating layer, 303: protective insulatinglayer, 310: thin film transistor, 311: gate electrode layer, 316: oxideinsulating layer, 320: substrate, 322: gate insulating layer, 323:protective insulating layer, 330: oxide semiconductor film, 331: oxidesemiconductor layer, 332: oxide semiconductor layer, 340: substrate,342: gate insulating layer, 343: protective insulating layer, 345: oxidesemiconductor film, 346: oxide semiconductor layer, 350: thin filmtransistor, 351: gate electrode layer, 356: oxide insulating layer. 360:thin film transistor, 361: gate electrode layer, 366: oxide insulatinglayer, 370: substrate, 373: protective insulating layer, 380: thin filmtransistor, 381: gate electrode layer, 382: oxide semiconductor layer,386: oxide insulating layer, 400: substrate, 402: gate insulating layer,407: insulating layer, 410: thin film transistor, 411: gate electrodelayer, 412: oxide semiconductor layer, 420: silicon substrate, 422:insulating layer, 423: opening, 424: conductive layer, 425: thin filmtransistor, 426: thin film transistor, 427: conductive layer, 450:substrate, 452: gate insulating layer, 457: insulating layer, 460: thinfilm transistor, 461: gate electrode layer, 461 a: gate electrode layer,461 b: gate electrode layer, 462: oxide semiconductor layer, 464: wiringlayer, 468: wiring layer, 580: substrate, 581: thin film transistor,583: oxide silicon layer, 584: protective insulating layer, 585:insulating layer, 587: electrode layer, 588: electrode layer, 589:spherical particle, 594: cavity, 595: filler, 596: counter substrate,601: substrate, 608: adhesive layer, 613: substrate, 622: light, 631:insulating layer. 632: protective insulating layer, 633: interlayerinsulating layer, 634: interlayer insulating layer, 641: electrodelayer, 642: electrode layer, 643: conductive layer, 644: electrodelayer, 645: gate electrode layer, 1300: photodetector, 1301: detector,1302: amplifier circuit, 1305: transistor, 1306: transistor, 1311: powersupply terminal, 1312: power supply terminal, 1320: protection circuit,1321: diode, 1600: cellular phone, 1601: housing, 1602: display portion,1604: external connection port, 1605: speaker, 1606: microphone, 1800:housing, 1801: housing, 1802: display panel, 1803: speaker, 1804:microphone, 1805: operation key, 1806: pointing device, 1807: cameralens. 1808: external connection terminal, 1810: keyboard, 1811: externalmemory slot, 2700: electronic book reader, 2701: housing, 2703: housing,2705: display portion, 2707: display portion, 2711: hinge. 2721: powersupply. 2723: operation key, 2725: speaker, 4001: substrate, 4002: pixelportion, 4003: signal line driver circuit, 4004: scan line drivercircuit, 4005: sealant, 4006: substrate, 4008: liquid crystal layer,4010: thin film transistor, 4011: thin film transistor. 4013: liquidcrystal element, 4015: connection terminal electrode, 4016: terminalelectrode, 4018: FPC, 4019: anisotropic conductive film, 4020:insulating layer, 4021: insulating layer, 4030: pixel electrode layer.4031: counter electrode layer, 4032: insulating layer, 4033: insulatinglayer, 4035: spacer, 4040: conductive layer, 4041: insulating layer,4042: protective insulating layer, 4100: photodetector. 4501: substrate,4502: pixel portion, 4505: sealant, 4506: substrate, 4507: filler, 4509:thin film transistor, 4510: thin film transistor, 4511: light-emittingelement, 4512: electroluminescent layer, 4513: electrode, 4515:connection terminal electrode, 4516: terminal electrode, 4517:electrode, 4519: anisotropic conductive film, 4520: partition, 4540:conductive layer, 4542: silicon oxide layer, 4543: overcoat layer, 4544:insulating layer, 4545: color filter layer, 4550: wiring layer, 4551:insulating layer, 4580: photodetector, 6400: pixel, 6401: switchingtransistor, 6402: transistor for driving a light-emitting element, 6403:capacitor, 6404: light-emitting element, 6405: signal line. 6406: scanline, 6407: power supply line, 6408: common electrode, 6502: transistorfor driving a light-emitting element, 6503: capacitor, 6504:light-emitting element, 6505: signal line, 6506: scan line, 6507: powersupply line, 6508: common electrode, 6510: pixel, 6511: switchingtransistor, 6512: switching transistor, 6513: reference transistor,7001: transistor for driving a light-emitting element, 7002:light-emitting element, 7003: electrode, 7004: EL layer, 7005:electrode, 7009: partition, 7011: transistor for driving alight-emitting element, 7012: light-emitting element, 7013: electrode.7014: EL layer, 7015: electrode, 7016: light-blocking film, 7017:conductive film, 7019: partition, 7021: transistor for driving alight-emitting element, 7022: light-emitting element, 7023: electrode,7024: EL layer, 7025: electrode, 7027: conductive film. 7029: partition,7031: insulating layer, 7032: insulating layer, 7033: color filterlayer, 7034: overcoat layer, 7035: protective insulating layer. 7036:planarization insulating layer, 7041: insulating layer, 7042: insulatinglayer. 7043: color filter layer, 7044: overcoat layer, 7045: protectiveinsulating layer, 7046: planarization insulating layer, 7051: siliconoxide layer, 7052: protective insulating layer. 7053: planarizationinsulating layer, 7055: insulating layer, 7056: planarization insulatinglayer, 9600: television set, 9601: housing, 9603: display portion, 9605:stand, 9607: display portion, 9609: operation key, 9610: remotecontroller, 9700: digital photo frame, 9701: housing, 9703: displayportion, 9881: housing, 9882: display portion, 9883: display portion,9884: speaker portion. 9885: operation key, 9886: recording mediuminsertion portion. 9887: connection terminal, 9888: sensor, 9889:microphone, 9890: LED lamp, 9891: housing, 9893: connection portion,1603 a: operation button, 1603 b: operation button, 315 a: sourceelectrode layer, 315 b: drain electrode layer, 355 a: source electrodelayer, 355 b: drain electrode layer, 365 a: source electrode layer, 365b: drain electrode layer. 372 a: gate insulating layer, 372 b: gateinsulating layer, 385 a: source electrode layer, 385 b: drain electrodelayer, 414 a: wiring layer, 414 b: wiring layer, 415 a: source or drainelectrode layer, 415 b: source or drain electrode layer, 421 a: opening,421 b: opening, 4503 a: signal line driver circuit. 4503 b: signal linedriver circuit, 4504 a: scan line driver circuit, 4504 b: scan linedriver circuit, 4518 a: FPC, 4518 b: FPC, 465 a 1: source or drainelectrode layer, 465 a 2: source or drain electrode layer. 465 b: sourceor drain electrode layer. 590 a: black region, 590 b: white region, 606a: semiconductor layer, 606 b: semiconductor layer, and 606 c:semiconductor layer.

1. An analog circuit comprising: a reference transistor; a mirrortransistor; and a detector, wherein the reference transistor iselectrically connected to the detector, wherein a drain and a gate ofthe reference transistor are electrically connected to each other,wherein the gate of the reference transistor is electrically connectedto a gate of the mirror transistor, and wherein the reference transistorand the mirror transistor include an oxide semiconductor having ahydrogen concentration of 5×10¹⁹ atoms/cm³ or lower.